What is a BCM or multiplexer?

[kilo3031]

I have a 98 Honda Accord that has a problem like many other people have posted. The dash and A/c lights turn off when the headlighs are turned on. It happened after I installed an after market stereo. After checking every fuse, wiring, and relays I could think of I found this forum.

The only solution that seemed to make sense is replacing the BCM (Body control unit) or multiplexer unit. This device(s) is supposed to be on the driver's side fuse box, but what exactly are these things. Doing a search for either of these items on google doesn't yield any good results.

Any ideas, part numbers, etc would be greatly appreciated.