Have a look behind your passengers speaker, there may be an amp/cross-over unit attached to it. The usual problem with the amp unit is that the power relay looses it suck (gets magnetised by the speaker magnet) and will not turn on the amp unit leaving you with no sounds. Replace the power relay and its all go again. By memory, if you trace the white with red stripe wire into the circuit board, you'll find the power relay.
This is assuming that you like fixing electronics?