Usually a parasitic load from the glovebox, underhood or trunk light on
the GM's...make sure that or a map light isn't that problem...close
them slowly and verify the light goes out...

If you don't find anything, you'll have to do a parasitic load test by
running a test light between the battery and a disconnected cable, and
pull fuses until the light dims (a small amount of load to the ECM and
Radio is normal), then check the manual to see what's on that circuit
and check all the items until you find what's staying on...

Put an ammeter on the battery, or just put a light bulb in series with
the battery so you can see that it's draining.  Go to the fuse box and
start pulling fuses until you find which circuit is draining the battery.
Then go look at what is on that circuit.

You need to identify where the problem is, then fix it.  Things break.
When they break, you fix them.
--scott