Why does my BCM only work when the headlights are on?

A few months ago the service engine soon light, the service light with the wrench icon & the security light all came on & remained on to this day. Along with that, the speedometer needle, gas gage needle & temperature gage needle were jumping all over the place & made fast clicking sounds. The odometer was tacking on 20 miles for every block I drove & soon the mileage was up in the 600 thousands. The shifter would not engage into reverse or drive when in park, but I lifted up the boot piece & manually lifted a lever that allowed me to shift it into drive or reverse. The AC went from blowing ice cold to blowing at room temperature. After researching the problem it sounds like the BCM but at night I noticed the car did not behave this way & everything worked perfectly fine, the gages, AC & shifter, although both the service engine lights remained on. I wondered what could be different in how the car operated at night as opposed to daytime that could make the car run differently & I figured out that the one thing different was that the headlights were on at night, so when I'd start the car in the day I'd immediately turn on the headlights & sure enough, the cars functioning would be normal & my odometer went all the way down to just 2,000 miles. If I turned the headlights off while the car was running it would go right back to functioning crazy! I figured turning on the headlights was somehow engaging the BCM & figured I had found some kind of bypass & it worked fine this way for about three months then I noticed the Airbag light come on & the ABS light (in addition to the service engine lights) but still, the gauges weren't jumping around, the AC blew cold & it shifted into reverse or drive as long as I turned on the headlights when starting the car. Then about a week ago, everything just stopped working, but now including the electric locks, the gas gauge, the speedometer and temperature gauge & all the warning lights remained on & I was back to manually engaging the shifter. After reading tons about the BCM's in L300's, I decided to disconnect the battery cable for a couple of hours & when I reconnected it, everything worked fine again as long as I turned on the headlights, but all the warning lights remain. The car always starts & has never broke down or died but once the electric locks, speedometer & AC stopped working I want to get it fixed. Now just yesterday, it wouldnt start at all, it would crank for a bit, then start to start but then go back to just cranking. I disconnected the battery for a couple hours, reconnected it now everything works again as long as I turn the headlights on when I start the car.

I hear BCM's are costly and. Can only be programmed by the GM dealership now that Saturn is no more. My question is, Is the BCM really bad because it still works as long as the headlights are on. Could it be a fuse or just that the software needs to be upgraded as stated in the recall on the BCM's for this car (bulletin 02-C-02 & 02-C-07 why TWO recalls?)

Thank you for your help, I appreciate it :)