If it’s cheap china stuff, it may have a voltage parasite. By that I mean the circuits aren’t actually fully disconnecting.

Installed a light bar.

If a sink leaks water when the nozzle is fully closed, you have a leak. Or in this case, your switch isn’t fully disconnecting.

Or you really screwed up and wired it backwards and have a short.

If the light bar has a cheap piece of frap switch, upgrade to a high quality switch, put that in the circuit, and leave the Crap one to on… that high quality switch is now your on off and if done correctly, won’t drain.

I am a mechanic but this is just information and is in no way a guaranteed solution.