"amp error" Radio stops playing while driving
 

The radio in my 2003 5 Speed protege stops making sound once I start driving. I have an aftermarket Pioner deh-x6900bt but the issue happens with the stock unit as well. I sit parked for 30 minutes daily and the radio plays and everything is fine. But when I start driving, within 15-60 seconds the sound just stops and the radio continues to show time elapsing in the song for a another 15-30 seconds and then the Pioneer reads "amp error." I tried a different Pioneer and had the same issue. I went back to the stock radio and the same thing happens (minus the "amp error" message)

From the time I purchased the car there has been no sound in the rear, only from the front two speakers but the radio has always worked.

I read the "amp error" issue could be a short with the speakers and I thought maybe the movement of the car was vibrating/agitating something, but when I bang around on the radio and all speaker locations while the car is stationary I cannot replicate the problem and it continues to play fine. I also shook the car from the outside and slammed the hood to try to replicate it but it continued to play. What gives? Also this may be coincidence but the day I started having a misfire is when this started. I then changed the spark plugs and one ignition coil. This couldn't be related to causing a short somewhere right?

Finally, I removed a rear door panel to inspect the speaker and found there was no speaker. Currently trying to find out if that is normal for this car, as the door panel has a speaker shape and everything but no speaker. The rear deck lid looks like it should have speakers too but I haven't pulled those off yet.