Both the brake light switch and dimmer switch contacts are supposed to be isolated from ground, but being down in the splash area, may have gotten corroded inside, which might cause some high resistance contact between the contacts and the grounded switch case. You could remove the wires going to the brake lights or headlights and leave the wire coming from the battery (or headlight switch) and see if the battery still draws down. Or just swap out the switches if you're convinced they're the problem area.
For your original question, the only way the headlights would be dim with the high beam indicator on is if you've swapped the connections at the terminal blocks on the inner fenders. The harness has a splice on the high beam circuit from the dimmer switch that powers the high beam indicator.
