What you can do is detatch the speaker connectors, remove the speakers and then test them by momentarily connecting to a 9V battery. The cone will move in or out. If it just moves, maybe thumps and does not bind or grind, speaker is probably good. If it seems to bind or grind, press gently on the cone with your fingers. be sure to push straight in and don't tear or dent the cones. This should confirm the grinding. What it usually is is the voice coil (a coil of wire) has come off the speaker cone's back. Time for new speakers. Save the connectors off the old speakers (you can de-solder them off) to put on the new ones - unless you buy speakers from Crutchfield. Crutchfield is a Class Act, they will give you connectors to use so you do not have to cut up your factory wiring.
If the speaker just thumped a little in the battery test, then connect it to a radio or some such and play some music. If the sound is not distorted, speaker is probably good. Then check the wiring, in as much as you can find the part of it that is not hidden. If your speakers are aftermarket look out for wire that got pinched and cut because it was run somewhere it should not have been run.
So, yeah, maybe it's the speakers. I'm suspecting it's a loose connector somewhere.
Oh by the way the rear speakers are supposed to be 8 ohms impedance. Most aftermarket speakers are 4 ohms, the factory radio tolerates that if you don't crank the volume up too loud. but if the volume is too high, with some program material the speakers may start to send induced current back to the radio and can mess it up. (For you electronics geeks - that's DC transients) That is, problem could be that the PO blew the radio playing it too loud, especially if PO also put in low-impedance aftermarket speakers.