I concur that it is probably the wiring or speaker voice coils. If you can find and disconnect a convenient test point, it is pretty easy to check continuity with an ohm meter. The impedance (a.c.) across the voice coil should be about 4 ohms, which means the resistance (d.c.) that your ohm meter reads should be slightly lower. Don't worry about the precise reading -- it is either going to be a few ohms (good) or something like 100K or even "infinity," meaning open (bad).
