Rear speakers in my daughter's '02 Pontiac Sunfire cut off intermittently. They may not work for a week or more, then come back on. There is no advance warning, such as static, pops or other noise. Also, I don't know if it's related or not, but the rear defroster blows fuses immediately when a new one is inserted. I'm not an audio guy, but I can use a meter.What should I look for first?
Check the wiring at the speaker terminal. Is it making a good connection? Check the speaker tinsel leads. Are they in tact (no cracks/cuts or loose)? Check the speaker wire in general (from HU to ALL speakers) for cracks, kinks, cuts, etc... Have your HU (head unit) checked out (qualified repair shop). Have your main computer checked out (qualified repair shop).
Last but not least.
IT MAY BE TIME FOR A NEW SYSTEM.
Your rear window defroster circuitry has a short in it. This = spagetti. A $39 diagnostic at a local non dealer car electrical shop should do the trick. If it is something small they will usually do it at no extra charge.