wiring is spaghetti

I bought an 89 mustang convertible for my son a few months ago. We decided this weekend to change out and upgrade the door speakers. The car has an aftermarket JVC head unit put in by the previous owner. When I pulled the head unit I found enough cheap speaker wire to rope and tie a bull. The factory harness only used the power connections for the head units connector. All the speaker wire connections from the head unit were connected with the cheap stuff. I am adept at using a DMM so we ripped out all the cheap stuff, pulled the door panels and went wire for wire to locate the factory speaker wires. Well we ohmed out the wires for the 3 inch dash speakers and labeled them. The door wires had been severed under the dash and bypassed with the cheap stuff but I managed to trace from the door to the head unit. At this time we have the dash speakers operating as "front" and the door speakers as "rear" (I don't believe the convertibles even came with rears) Now for the fun part. I found 4 pairs of wires stuffed behind the heater core. I traced them to an amp under the passengers seat. To say we were suprised would be an understatement. The amp was definately installed by a professional prior to the neighbor I bought the car from. And now for the question........the only place to mount speakers in the rear is the side panels which is where mustang coupes normally have them. I would like to use the amp for a set of 6 1/2 inch rears since they end up covered by the seat. Are there any good speakers that size that can handle a 150 watt sony amp? Thanks for any help you guys might offer.... ps: I did suffer a headache after 4 hours of wire tracing....

We just moved the old door speakers to the rear compartments. I reconnected the blue wire from the amp to the back of the head unit, plugged in the RCA plugs from the amps rear to the head units rear and turned the whole thing on. the only way any sound at all comes from the rear speaker is if I unplug the white RCA and touch the tip to the outside (ground) part of the amps connection. It's an odd deal indeed. Right now, without considering the amp, the head unit is wired as it should be. The only oddity is that the rears are actually the doors and the fronts are in the dash. I'm not that familiar with amps as much as I am head unit installation. I suppose I could always just wire the back seat speakers in parallel with the door speakers for sound but I know OHM isn't fond of that arrangement. I have my son putting everything back together for now. At least the system sounds better then it did before since one dash speaker (infinity) wasn't even connected. The last person was a real butcher with the wiring but I have it pretty well organized now. Should the amp be able to power the rear seat speakers alone? In essence there would be 4 rear speakers. Two off the amp using the rca's and two off the regular head unit plug. And I thought replacing the bose system in my vette was rough.....nothing compared to this....