Battery draining, why?

for more clarification my all interior lights still power on, and the exterior lights work too, but the headlights are getting dimmer each time I try, I think... Also when i say the ground is a 3rd of what it should be, I mean the connective point on the end grounded to the top of my trunk has been cut to a 3rd of its original thickness. Any one with a solid opinion of what's going on is encouraged to answer! please!

Used the proper harness, popped the hood to disconnect the fuze and realized... the power cord was torn out of the fuse holder for the amp leaving the fuze connected to the battery without being connected into the cabin of the car.. Since the part that controls when to shut off the power being drawn is in the cabin and not connected is it safe to assume that this cord with fuse piece attached to the battery drained it?

I just completely did a massive overhaul on my audio in my pick up truck with a sub that goes through the back tinted window for the drive in movie. One of the things the tech did was put a power surge block about 2 feet from the battery. Sure, I have an amp, too, for the sub, but no battery drain issues. Good luck PS, that power block had a washer on the end of it, o ring connected to the pos terminal