I can let my 87 sit in the winter for easily over a month and it will still start.

If you put a meter to the battery, it should show the draw from the load that is happening. Then start pulling/replacing fuses one by one until the load drops. At that point, you know which circuit it is and can hone in from there.

Big Truck: 00, 3.5, Endeavor, 5-Spd drive line in hand!
Little Truck: 87, 2.6T I/C, MT, LSDs, Tonneau Top
Her Truck: 03, 3.8, 20th Anniv, 65k
Daughter's: 06 Eclipse, Keeping it Mitsu!
FSMs: MitsubishiLinks.com