I've got two '99 Gt's and they both have issues with the rear speakers. One one of them I can't get anything from the rear passenger speaker. I think it needs to be replaced, but should I check for loose wires?

The other car is trickier, the rear speakers sometimes work. They'll be fine for a few days and then nothing. A few more days and they work again. I'm don't even know where to start looking for the problem.