2002 Nissan Frontier Battery problem

2002 Nissan Frontier (4cyl, 2.4L) Couple of weeks ago i left my lights on, got a jump, no problem for 2 weeks. then one day i try to start truck, it acts like the battery weak but cranks. take my girl to the movies. go outside the theater and truck wont start, little power, beeps with key in ignition then battery goes, completely silent no beeps or nothing, got another jump, drove directly home. next day bought new battery, install and battery is dead from time i connect cable and walk around to start. little power left in batery, return battery, checks out battery only has 10.5v, money back in hand travel for different battery at different place. Install, battery is very weak but jumpable. with engine on disconnected battery cables. engine still running, (the only way i know how to check alt). I think i must have some crossed wires polarizing the battery, or draining it, gounding out or something. could this be ECM (please no)Please help with ideas for do it yourselfer
New problem, new battery immediately drains after installation, took battery out of different vehichle and installed, only few beeps and no engine turn over, put battery back in original vehichle and battery starts vehicle immediately, took off alternator and took to parts store, tested it and said it was fine, took it to different parts store and said it was borderline and not putting out enough volts. Its not the battery, its not the alternator, could it be a switch or fuse, is something being left on to cause this problem.
Desperately need help


katzenjammer

please help with this response do it yourselfer katzenjammer