i have a les paul and when i play with the treble pickup it plays fine. however, whenever i switch to the rhythm pup it gets REALLY soft and dirty sounding. i'm not really sure if the pick up is simply shot or what is going on because it still works to a very limited extent (sounds like crap but sound still comes out) maybe it's the wiring? or something screwy with the toggle? just wondering if anybody had experienced this or had an idea of what might be the matter...
Do you have a multimeter you can use to test the resistance of the pickup?
Then you are going to have to play a bit of guess and check. Thy soldering your bridge pickup to where your neck pickup is supposed to be soldered. Then see if it still does it. Let us know what you find out.