check the speakers they may need re-foaming. If not look at the electronics and look for burn marks from components burning out, fuses blown, and SMELL. One thing I have found is that a slight whiff can find the problem. If it smell burnt you definitely have a problem with burnt out components even if you can't see the burn marks.
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if your getting signal to your speakers and it's fuzzy you may have a ground loop. try running a ground cable back to your head unit ( whatever feeds the amp) and connect it to the same place your head unit is grounded at. It may (or may not) be a screw stud. if there is know ground run one to the chassis ground.
This is one of a few possible fixes good luck.
Are the speakers around ten years old? Is it possible sunlight fell on them through baffles and grills while the car was parked outside day after day? If both of these are true, you are probably due for new speakers. Even if only one of these is true, you probably could use new speakers. The difference will amaze and please you.
My best suggestion is taking your speakers out and hooking them up to another source. Listen and see if they are sounding bad. Then, to test the signal from the amp, hook up a different speaker in place of the car ones and listen to them. I have done this, and this usually gets the job done.