Got myself a '98 ES300 last week- It's got 7 speakers, an amp and head unit that's all intertwined. Metra makes a Toyota adapter that is the solution to using an aftermarket head unit with the existing speakers & amp. I bought one and spliced it together with my Empeg adapter, everything matched fine. It takes every wire and puts it together in a single harness, so it has RCAs for LR/LL FR/FL.

It fits and I can turn the Empeg on, but i get tons of garbage sound with the music. I think it's a ground loop..

So, I ran the Empeg its own ground to a bolt on the frame (scratched off the paint, too) instead of using the adapter's ground. No difference.. so I ran the empeg its own power from the battery, no difference. Grr. So I unplug the Empeg RCAs from the adapter, and got a RCA-> headphone converter. Plugged headphones into the Empeg and it sounds fine! no noise. fscking weird...
So, using the adapter for power but not the adapter's RCAs I can hear it fine (through headphones). But, if i plug it back into the adapter for the amp, i still get a lot of crap sound with the music. I put the stock head unit back in and it sounds fine., no sound artifacts at all.

Anyone have a clue? Maybe I have bad RCAs or wires on the audio part of the adapter? i'm stumped, i tried the obvious.. and the leads on my empeg harness are getting shorter by the day!