My '94 wagon's stock front speakers work fine, but the stock rear speakers (doors and rear tweets) work for about 10 seconds when I start the car in the morning, then quit the rest of the day.

I've checked the spade connections at the speakers. All magnets are attached. Sprayed contact cleaner in the fader pot. I've 'jiggled' the wire looms in the door jambs and the head unit (sc-811, there is no separate amp in my car).

ONE HINT: all those rear speakers work and quit at the same time, not just one or two are out.

Sure sounds like a ground connection or the head unit to me.
Any ideas what to do next? Is there a common ground or place where the wires might get damaged? Someway to check the HU? Thanks!