I have a fourth gen. and I generally stick to the fourth gen. forums, but my friend has a third gen with a problem. He thinks he too good to sign up for an account so he is using mine. Here goes...
His stero system have been giving him problems ever since he bought the car oh about a week ago. His front passenger and rear driver speakers don't work. They're not blown becuase we took them out and plugged them into a home stero system. What could it be? I checked the stero with the original Bose put back in and same thing. Any help appreciated.

 

haha gunna sound obvious but hows the fade set. if its completely to the rear it is known as a DEE DEE DEEEEE situation. my friend had that with his but it wasnt a bose deck it was a tape deck. he didn't realize u need to pull the pin out till it clicks to adjust the fade. are there any loose connections? wires cut along the lines?

 

it cld be the radios resistors are burnt out thats why he has the right side working and not the left.

 

do they crack and then go out, is there no sound ever?
I don't much about all 3rd gen systems, but if it's set up like a 90se, the speakers have a seperate amp each, and they tend to go out sooner or later. I didn't really understand the point of the Bose system design, so I swapped out the system/speakers to make life easy. I think you can just put in a new HU and resolve the issue (and since you got sound when you plugged them in to a home stereo that makes me think this will work) but am not sure if you need to bypass the built in amps, yadda, yadda - someone on here should know better than I.

 

I would say the amps?

 

I had a problem like that with my system too. I took it in and found out it wasn't the speaker, but it was the speaker amps. They guy bypassed the amps, which is when I found out the stock stereo only sent out a weak signal (the reason for the separate speaker amps). I eventually wound up replacing the deck and the speakers (amps bypassed of course) and now everything works fine.

 

falseicon is on the right track.

Each Bose speaker is individually amped. If you do some digging, you can find some ways to fix the amps.

In my experience, they all become problems eventually. The amps aren't very well made and the speakers are all cheap paper, exposed to extreme heat and cold. By now, most of them are in the neighborhood of 15 years old.