Hello BB,
I have an '86 245 that is methodically blowing rear speakers. With my first aftermarket head unit two sets of 6x9s blew. The speakers did not blow together as a pair but one at a time separated by a week or more. The system is not amplified and the speakers were 45 - 55 watts RMS, 150 watt max. The head unit was 4x18 watts RMS, 4x50 watts max. The front 4" speakers have never blown (45 watts RMS). I physically inspected the rear speaker wires (18 gauge) for kinks/cracks/breaks etc., and found nothing on either side (I had installed the wiring new with the system). I probably should have checked continuity but the wires are clear and again I saw nothing looking remotely suspicious. I replaced the head unit (4x22 watts RMS, 4x50 watts max) and got a 3rd pair of 6x9s (80 watt RMS, 400 watt max) because I figured it had to be the head unit. Within a week I had blown another speaker! I have since checked the stereo ground wire as well as the other grounds on the center console. I have to wonder why it is only happening in the back - and now to a 400 watt max speaker! The main difference between the front and rear is that the front speakers are mounted in the doors while the rear speakers are mounted in boxes. I am basically stumped. Any ideas out there?
Thanks,

Mike