Ford Repair: battery drainage or battery warning light, ford bronco, 351w

Question
i have a 89 ford bronco with a 1993  351W.  everything with the engine swap has been fine.  when i bought a new battery the other day, my brother put the battery in backwards.  after i heard his hollers for help, i saw smoke coming from the starter or neighboring wires (couldn't quite tell in the hurry to disconnect the cables).   come to find out (i think) only the wiring from the starter to the alternator was burned.  after correcting the mistake and reconnecting the unburned portions of the wire with a new connector to the starter, it cranked fine, but the battery was dead by the next day (although it could have been from a superfluous drain from an interior brake light).  after having the battery tested, they said it was not rechargeable.  after getting a second new battery (and two alternator checks which came back okay), i dropped the battery in and cranked her up.  the battery warning light came on.  after driving her around, it seemed to be okay but the light wouldn't go off.  the next day, after cranking, the light was not on, but my battey gauge had dropped significantly.  what's draining the thing?  any insight would be much appreciated....  

Answer
Hello and thanks for the question...I'll try to help.
 What I would suggest is that you start checking for the amperage draw by connecting a amp/volt meter between the battery terminal and battery cable end. Then start pulling fuses, one at a time, and also disconnect things like the starter, alternator. When the meter reads 0.03 or lower you found the circuit that the draw is on. Stop there and fix it.
Hope this helps,
Erik