Aftermarket Head Unit Installation Problem

[bigfuz]

Hey everyone. This is my first post here and I hope you guys can help me out. I also apologize for the length of it, but I want to give as much information as possible.

I've done some searching around but haven't found an answer to this admittedly strange problem. I'm trying to install an Alpine CDA-9886 radio into my 1998 GS400. I'm using the Metra 1987 and later Toyota wiring harness (part no. 70-1761). This harness has two connectors; connector 1 has the red, yellow, black, and gray/white wires for power and front speakers, along with solid blue for power antenna(which I assume I'm not using because there is no external antenna). Connector 2 has the purple and green rear speaker wires, along with orange illumination (which I'm definitely not using as the Alpine has its own dimmer control). The connectors fit exactly, and the problem I'm having is that the radio is somehow being powered via the rear speaker leads (purple and green). When I first installed it, the radio would not power up unless I had the yellow wire from the radio wired up along with its red wire to the red wire on the harness. In that configuration, when I unplug connector 1, the radio stays on, all functions normal, the only connector connected being no. 2 with the rear speaker wires. As you can probably imagine, it's giving me a complete brain melt, as I usually consider a radio not able to be powered by its own speaker outputs. As expected, throughout this experiment, I have had no sound coming from the radio as well. I've never encountered an instance like this, and I have previously installed this radio into my old ES300 using the amplifier integration harness with the RCA leads on it. I'm stumped so far, so any help is greatly appreciated.

[1990LS400]

I'm not much of an audio guy but maybe the attached diagrams for the standard Pioneer in the 1998 GS400 might help? You did have the standard audio system, didn't you. I don't remember seeing adapter harnesses for cars equipped with Nakamichi but they might be out there.

1998 GS400 audio diam 1.pdf

1998 GS400 audio diam 2.pdf

[bigfuz]

I do have just the standard Pioneer system. No navigation either. I've seen the diagrams before, but not sure what I can do with them except for a straight splice to the Alpine.