Ive been having issues with my montes battery going dead while it sets for the winter, ive been threw 3 batteries already luckly the battery place has replaced em but within a week it goes dead, so i took it to the local shop to check it out & they think its my bcm, does this sound right? & if so whats a new one cost? Ikn i can swap it myself

 

Did the shop check for parasitic drain? If the system continually drains fresh batteries that would be the first thing I would do.

 

The BCM controls EVERYTHING! Lights and all. Switches tell the computer to turn the lights on. Yes, it COULD be that. But are there any other symptoms? Clicking noises? Flickering lights? ANYTHING unusual?

 

I've been having issues with my Monte (driven only during the summer) that if it sits for about 3 days, the battery is dead. This year I had a battery/alt test done and the battery failed. Replaced it and it's still doing it. Next spring, I need to do a parasitic drain test. I hope it comes back as something minor.
In addition, ALL the electrical is bone stock (minus my change this past year of bulbs to LED lights, but my drain problem has existed since I've owned it in '08).

 

They have been checking it all out going over it one thing at a time & the head mechanic says it prob is that, i dont drive it enough to notice things goin wrong with it much, wiring is all factory on mine except my amp & sub but i undone the power wire after the 2nd battery went dead so ikn thats not it, within a week a fresh charged battery is dead, so i have no idea

 

My 01 is doing the same thing after it sits without starting for more than a day. But I did notice a clicking noise around the passenger side of the dash. it starts clicking after i have tried to start it and the battery is drained. Figured i would throw that in, maybe it will help ya.

 

I had this problem with my Monte a while back. In my case, it was the amp draining the battery, because it wasn't properly hooked up to the ignition. I had it fixed so that the amp completely turns off when the car is off.

Although, you said you looked into the sub/ amp...so IDK.