I bought my car used a month ago and my dash started (2 weeks ago) to show my highbeams were on. When I try to turn them off and on I don't see any change in brightness.

A few things (that may or may not matter):

• I have no wheel well liners or underbody covers at the front due to an accident (I had to pull them off). Idk if this matters but maybe all the slush/salt from winter has messed up some wiring somehow?
• I discovered today that the positive terminal on my battery is completely loose (it comes right off with no force)
• The battery itself is in good health
• The headlights are using aftermarket bulbs (pretty sure they're LEDs)
• Starting today the left headlight will decide to turn off randomly
• The highbeam indicator will show weakly even if highbeams are off; will show properly if when I turn them on

Any ideas where I could start diagnosing and replacing things? Lmk if pictures/videos would help.