i got a friend with a 2000 cummins and the heated seats, and passenger speakers don't work. the heated seat lights don't come on and neither the front or back passenger speaker work, but the driver side is fine

 

fuses or a bad/loose connection somewhere

 

is the control balance for the stereo turned all the way to the drivers side? Just checking

 

ya i've played with all the controls and checked all the fuses, and came up with nothing still. i even pulled the driver side door trim because the two brown wires in there are for the passenger side speakers, and all the wires are in great shape.

 

You can check the speakers by taking the panels off and hooking a 9V battery to the speakers and see if the voice coil bounces the woofer. If so the speakers are working. If you have the infinity system the front speaker will have an amp connected to the 6x9 speaker you can remove the amp and test the speaker after the amp. Maybe the entire left channel in the stereo head is bad. You should be able to check for 12 volt signal on the seats on the outgoing side of the heater seat switch. If there's power the heater in the seats may be deffective. If no power start tracing it back from the seat. You can also get on All Data.com and pay 20 bucks and get a pretty good breakdown of the wiring for your truck along with other info. Hope this helps