what is this? stereo wiring question

[>X<]

yeah, well i had a urge to do some work on truck today, so i finished shaving front side markers, started on back ones, took ot glovebox and took out crappy tape player (see other post about friggin dash) because i'm about t do something to dash, dont realy know what yet though.

BUt anyway, i was labeling the wires for the stereo and i am stumped. theres are two wires that go from back of stereo, spliced n to speaker wire, then split apart again, one goes t fusebox luster of wires and one goes to this thing that is near my gas pedal. when you push the pedal in it hits this plastic thing that either cuts off or gives elec. to this wire, that goes back to stereo. what the hell is this? a volume control, maybe turns it up? i never ad any speakers in so i dont know what it does, radio does work though. an thoughts? i took soem pictures but they came outbad so i'll get some more in a minute. thanks, sean