Need help with my 2012 Sierra

I have a 2012 GMC Sierra 1500. Stock audio system but I just installed a Jensen 600w amp and an old MTX 10” I had laying around. I used a LOC to tie into the factory rear door speakers and everything works good. The problem is now the truck is reading that the rear drivers door is ajar and it kills my battery. If I disconnect the tied in speaker wires the problem goes away. I don’t have the money for a new head unit so I’m searching for any other solutions. Thanks in advance!