Extrudedco

Yes, my username is a typo I didn't bother to fix....

So after some experimentation, I've found and implemented a mod to give me a direct audio input into the Nakamichi head unit in my 1991 LS400. This mod hijack's the cassette output. It still requires a tape of some sort to keep the head unit in cassette mode, so I'm using a tape adapter that was malfunctioning. There appears to be some audio bleed over into the other inputs for some reason, but pausing the mp3 player eliminates any interference in my limited testing so far. Edit: now that I think about it, the head unit shuts the cassette component off when you change modes, so the bleed through effect to other modes may be due to how the unit is designed.

My first attempt tapped into the wire harness just after the volume ****. This had several disadvantages, such as not being able to control the volume with the head unit, and a self inflicted stab wound with a screwdriver while forcing modified pins back into a plastic harness. I do not recommend this.

Nakamichi had some foresight in the design of their head unit. It appears that any time an audio signal is passed over a wire harness, they used the same easily identified wire pairings, a pair of white and dark grey wires that have short lengths of white or red heatshrink on them. The red heatshrink indicates the right channel, the white indicates the left.

So to the mod itself. You'll need to pull the head unit (obviously) and remove the top and bottom panels (the rear may help too) so you can remove some of the guts. What you're looking for is a small PCB that's next to the cassette unit. There's a small metal bracket (very narrow but runs the length of the pcb, fits into slots on the PCB to hold it in place) holding the circuit board, remove the two screws and the bracket. Unplug the 4 pin connector near the front of the circuit board and lift the board out. Now unplug the 7 wire harness pictured below. (You may be able to remove this harness without removing the circuit board at all) The audio wires in this harness are what we are looking for. With a small flathead screwdriver, gently lift up the plastic tabs on the plug and remove the 4 audio wires (dark grey and white) from the harness. Grab your audio cable of choice with one end of the wires stripped. Run the cable up through the back bottom end of the head unit to take advantage of a convenient gap. Solder (not strictly necessary, but recommended) the stripped wires to the wires you removed from the harness. (Dark grey wires are the grounds, white wires are the signals. The wire pair with the one inch long red heat shrink is the right channel, the white heat shrink the left) Insulate the exposed wires with electrical tape or heat shrink tubing. Reassemble the unit.

I ran the wire under the shifter and up through the hole where the wiring for the phone used to run, so the cable is hidden.
Update: It's been working great! It looks like I may have patched in after the tape signal is amplified, so it's not quite as loud as the original tape adapter was. No audible static, and only a faint hiss when I crank it up all the way, so music is very clear now.