Grounding Troubles

My sister just bought a '92 Maxima, I think its the SE model with the twin cam V6. It has one of those factory Bose stereos in it, but the guy who sold it to her told her that it hasn't worked right for years and he thought the speakers were blown, since only one plays. When she was going to school today, the speakers started playing an unbearably loud single tone that seems to come on and off with the fuel pump (sometimes its a quiet static, sometimes its a loud tone). I'm assuming that something isn't grounding properly back there, but I'm not sure where to start looking. Does that sound like I should start with the speakers or the fuel pump? The noise stops if I shut the radio off.

Also, does anyone know if there's an access hatch for the pump under the back seat?

Start with the amplifier. Not sure where it is, but the Bose system has an external amplifier. The symptoms you describe are more typical of a blown amplifier.

I would simply pull each speaker, and the head unit out, and check all the wiring first.
Its possible something is shorting or grounding when it shouldn't be.