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Flip the dash switch, your Bluetooth audio device connects automatically to your old car FM radio.

After years of searching for a pre-1998-facelift Lexus LS400 in nice original condition, I found this 1997 Coach Edition in Silver Jade Metallic and snapped it up. It's an amazing car, nearly 20 years old and completely problem-free, but there was one problem: the factory audio system sounded great but played only cassettes and CDs. While cassettes were fine in their day, I prefer to play digital-format music files from the 100 gigabytes of MP3s on my smartphone. Even my hooptie 1992 Civic has Bluetooth connectivity for its junkyard audio system… but I didn't want to violate the numbers-matching originality of my Lexus, not by swapping out the radio and certainly not by drilling holes in the dash. Here's how I solved the problem cheaply and without doing any modifications more serious than splicing a few wires.

Warning: Do not attempt any part of this project unless you already know how to work with automotive wiring, read a wiring diagram, and use tools in a safe manner. Screwing up this stuff can result in sadness, misery, injury, death, or worse.

The MIL-BTREC Bluetooth dongle has no controls; it powers up and seeks connections as soon as it gets 12 volts DC.

Wireless FM modulators suck, and we will speak of them no more here. What you want, if you need to feed a quality audio signal to a car FM radio that lacks an AUX jack (i.e., just about every factory radio made before the late 2000s), is a wired FM modulator. This device goes between the car's antenna and its radio and feeds a signal to a designated frequency when it's powered up; I have found that the $20.35 Scosche FM-MOD01 unit gets the job done just fine. This device is made with a couple of female RCA jacks, but only suckas run an audio cable from the phone to the modulator. That's where the MIL-BTREC marine Bluetooth dongle comes in; it has no controls of any sort, and thus no maddening little buttons to push. It boots up when it gets power, searches for paired Bluetooth devices, and feeds the resulting audio into RCA cables. Connect the two so they receive power from the same circuit, plug the dongle's output into the modulator's input, and the whole mess can be hidden under the dash and out of sight.

I have no idea what optional switch Toyota might have put in this under-dash opening, but it's a perfect location for my audio devices

Since I was not willing to drill or cut any holes anywhere in my Lexus, I had to find an existing switch location. On most cars, you'll find a bunch of blocked-off or covered holes in and under the dash, where options not purchased for that car were intended to live, so I hunted for those. You go to the junkyard, find the switch meant for the empty hole in your car, and it pops right in. The LS400 had few options, being a high-end luxury car, but I found this opening under the dash, next to the data connector and the pushbutton switch that turns the door-lock beep sound on and off. A little investigation showed that the beep switch used an identical opening, so I headed to the junkyard for some shopping.

1995-1997 LS400s are just starting to show up in the big self-service yards, so I was able to find this one.

Early LS400s are commonplace in the big self-service wrecking yards that I frequent, but most of the parts from the 1990-1994 cars won't fit my '97 and junked 1995-1997 cars are much harder to find. Fortunately, this fairly complete '95 LS400 was awaiting me at a yard close to my house.

A little prying with a flat-tip screwdriver, a little wire-cutting, and I had the door-lock-beep switch from this car. The junkyard cashier just laughed when I tried to pay for it. Price: free.

The original door-lock-beep switch is on the left. My new switch is on the right.

I soldered some long leads onto the switch wires and popped the switch into the hole under my dash. It looks like it was meant to be there.

I didn't want the power antenna to stick up while I used the Bluetooth source to the radio, so I had to get an SPDT relay for an antenna cut-out circuit.

One minor problem with using the LS400's radio for my audio signal was that the power antenna deploys automatically when the radio is turned on. Since I didn't want the antenna poking up, subject to damage, when it wasn't needed, I decided to use a relay to interrupt the antenna-actuation signal wire between radio and antenna. You can go to any wrecking yard and find hundreds of Bosch SPDT relays in European cars (I prefer the Bosch type because most will have the circuit diagram printed on the case), or you can pay $3.65 and order a new one.

Here's the schematic I drew up to help me get the wiring straight.

It should go without saying that you need the factory wiring diagrams for your car to do any of this; sometimes you can find them online, sometimes you have to buy a used factory shop manual on eBay. What I wanted to do with my relay was to have the relay's 87a contact connected to the 30 contact (these are standardized relay-contact numbers, using DIN 72552) when the FM modulator and Bluetooth dongle were not powered up, and for that 30-to-87a connection to be interrupted when power was fed to the relay (via the same circuit that powered the modulator and dongle). In my crude schematic, above, you can see that the car's accessory power (via the junkyard switch) feeds the relay coil (contact 85) and both the FM modulator and Bluetooth dongle.

I got busy with the wire and the crimping tool, recreating my diagram in real-world wiring and connectors.

Always fuse electrical hardware in your car. Fire=bad.

Even though the accessory circuit I'd be tapping for power to my new devices is fused, I didn't trust that fuse to blow if one of the skinny wires feeding those devices got grounded by mistake. So, I dug into my stash of random electrical parts and found this fuse holder (if you don't feel like grabbing a fuse holder and fuse at the junkyard, you can invest $3.39 for a new assembly). A 7.5A fuse should be about right for an FM modulator, a Bluetooth dongle, and a Bosch relay coil. I rigged the fuse to the power circuit for these three devices. Always add a fuse to any new electrical circuits you put in your car's dash— having experienced a couple of car fires caused by bad wiring, I say this with conviction.

Now the radio comes out.

Now comes the scary part with a car this nice: removing the dash trim and getting at the wiring harness behind the radio. The LS400 has plenty of nice wood and hidden fasteners, so this job took me a while. If you're going into a hooptie's dash, just jab a prybar in there and get busy.

I had to cut two wires in the radio harness.

Using the factory wiring diagram for my car, I located two wires in the radio wiring harness: the wire that activates the power antenna, and the wire that provides power in the AUX or IGN positions of the ignition switch. I ran the leads from the 30 and 87a contacts on my Bosch relay to the antenna-power wires, so that the current to the antenna would be interrupted when the relay coil was activated. Then I tapped into the power wire I'd just cut, so that the FM modulator, Bluetooth dongle, and relay would have power only when the radio had power.

These wires are the only real modifications to the car, and the LS400's radio harness is a stub unit that can be replaced later if necessary.

Some work with butt connectors and the crimping tool and it's all hooked up. The car's originality has been compromised only slightly now.

Nothing permanent here, just zipties holding the components out of site under the dash.

Once all the wiring was in place (including the Motorola antenna connectors on the modulator and back of the radio), I took the FM modulator, Bluetooth dongle, relay, and fuse holder and tied them together with zip-ties. The whole clump of parts went up and out of sight under the dash, with more zip-ties holding it in place. I connected the ground leads from the devices and attached them to a ring connector screwed onto a handy grounded fastener under the dash.

It works great! When the pushbutton switch under the dash is in the OFF position, the radio and power antenna work normally. When I push the button to the ON position, the power antenna retracts, the FM modulator comes on instantly, and the Bluetooth dongle (which I had already paired with my smartphone) connects soon after. The entire sequence, from pushing the button to music from my phone playing through the radio, takes about five seconds, and the sound quality is excellent.