Hi,
I would think that it's a loose wire. You can get to it by taking off the speaker grill and the speaker. It's pretty easy. If you have the type of speaker I have on my wagon they should be next to the rear seat. I know there are many variations on the wagon speaker systems, and this might not be yours. I think the easiest and best sounding way is to run an amp to two 6x9's in boxs in the back. It doesn't have to be a powerful amp. I have a wal-mart cheap 250 watt amp connected to my sub and it sounds good. I have tried hooking the amp to 2 6x9's and it sounded amazing. It produces a better and clearer sound. Also I replaced the 4" front door speakers with 2 Audiobahn 2 way speakers and that helped the sound a lot. If you do that you will be very pleased with the results. I like to use my car for music whenever I'm at a picnic or BBQ because it sounds so clear. The new speakers also can go louder than the stock Volvo speaker without distorting.

Charles 1985 245ti 134K
2 Audiobahn 4" speakers, 2 Blaupunkt 3 way 6x9's, 1 Rockford Fosgate 10" punch HE Subwoofer, Jensen 250 watt amp, Blaupunkt 230 watt CD player