I recently just bought a 1994 mustang gt 5 speed with 92,000 miles its has the mach 460 radio installed. The radio turned on but there is no sound at all. Until today. I went to get in my car turned on the key with my door open and there was sound out of the back speakers but not the front. That is until i shut the door then poof no more sound. I've checked all fueses and I've checked that the amps at the rear have power with a test light.
I agree with General karthief, it sounds like a wiring issue — however, to cover as many bases as possible it's also potentially a component with a loose internal connection.
In situations like this, I find it helpful to do a printout of the wiring diagram and trace the positive wiring with a red marker and ground with a green (any two colors will work). Then go and find that specific routing in the car.
A few years ago I had an issue in my Saturn with the computer intermittently throwing a code for the EGR valve. The ASE Master technician working on it at the dealership just replaced the EGR valve on the car and when it didn't resolve the issue, tossed up his hands and said "I don't know." I took the time to trace the entire EGR circuit and found the clip holding the clutch pedal on the carrier shaft had sawed its way through the wiring harness, causing the intermittent issue.
You never know how random, unrelated, and deep in the weeds these kinds of issues can be.