My heater drains my battery when it is not running?

Okay, so a few days ago I went to start my vehicle and the battery was drained. Weird okay, charge battery I go about life as usual. Until later that evening when I went to run to the store and I opened the driver side door and realized I heard the back heater wigging out, going off and on at random, and it was completely powered off. As a matter of fact, I made sure all the knobs were turned to the off position and it continued to go off and on. I have no clue what's causing this but clearly it needs to be addressed. Thanks!