I have a 2004 Subaru Legacy wagon with a pioneer head unit that is 4.5 years old with factory speakers. I have made no changes to the system or setup, but earlier this week only my front left speaker was working. I parked my car and when I started it later, all speakers were working again. A couple days later, I started the car and only the front left speaker worked. It seems like they are either working or not, no cutting in and out, no distortion.
I checked the head unit connection and everything seems to be fine. I removed the passenger door panel and the wiring to the speaker looks fine. No frayed wires or bad connections.