Lighting issues when headlights are on

[Pdempsey]

Hey everyone, new to the forum. I'm having an interesting problem on a 2006 Silverado 2500HD that I'm looking to buy as it's the only problem with the truck.  So when the headlights are off everything works like it's supposed to, but when the headlights are on, the turn, reverse, tail, and parking lights do some weird stuff. Like if you hit the brakes the turn signals on the dash and on the mirrors light up faintly. If you turn the left or right blinker on they wont blink if you're holding the brakes and when they do blink they both seem to be lighting up, one just brighter than the other. Another odd thing is if you turn the hazard flashers on, the turn signals barely flash but the aftermarket reverse lights mounted in the bumper flash, but the reverse lights in the actual tail lights dont flash with them. Turn the headlights off and everything goes back to normal and works like it should.  I'm sure I'm missing some symptoms but that's the gist of it. According to the owner he has replaced the headlight switch, and has swapped the turn signal switch and the distribution block mounted at the rear of the truck with others from another working vehicle to no positive effect.  He said he thinks it might have something to do with the BCM but hes not sure. I'm a heavy equipment tech but have very little experience with vehicle electrical so any help is appreciated. 

[riverbanks]

Possible ground issue,locate ground on frame for rear lights,on my 02,it's on frame/front box support near filler pipe,2 there