Installed new in-dash, lost sound from door speakers
 

I used the standard Toyota wire harness to install a deck on my FR-S. it treats the speakers on the dash as the fronts and treats the back seat speakers as the rears. The door speakers are completely ignored.

There is a third harness that doesn't get used. This must have the door speakers and maybe an amp on it.

Just an FYI for anybody who wants an aftermarket radio. It sounds pretty bad with no door speakers.

Interesting. I think I saw the amp for the speakers in the trunk, under the "floor".

When I had my 2000 Celica with the JBL system, the external amp needed RCA inputs when adding an after-market radio. Does your volume seem correct? Or is it too loud? If it's too loud, maybe you can modify the plug for RCA connection?

It will be a few months before I change my radio out (I need to find a good one). So right now, I'm no good help. Just theories!

Yep, the amp in the trunk is indeed for the door speakers. I didn't hook up the power antenna wire because I figured there was no reason to, but that wire happens to be the remote wire for the trunk amp. Hooked it up and now all the speakers work.