1998+ s10 and blazers have a BCM. the PCM is under the hood. it stands for powertrain control module and controls the engine and transmission. the BCM is under the heater vents at the bottom center of the dash, it stands for body control module and controls all electronic functions including lights.
the BCM can recognize that the lights are on with the key off and turn them off after a few inutes to keep the battery from going dead. it can tell if you leave the door open and shut off the dome light. it uses low current signals to trigger high current signals at distributed electrical boxes throughout the truck to minimize the amount of large gauge wiring needed. for example, you turn on the light switch, but its only a small current wire that sends a signal to the BCM, the BCM sends a small current signal to the underhood distribution and turns on a high current supply to the lights. think of it like a middle man, all requests go to him and he decides what to do about it, but doesnt do any of the work himself.
it also controls your PASSLOCK system, telling the PCM its ok to start the truck. if you have a BCM problem you need a new one, there is no way to reprogram a used BCM to match your VIN.
