headlight brake light issue

[fiddz1959]

I bought a 2005 GMC Sierra 2500hd 5 months ago. I had a brake light (right side only) issue on my holiday trailer, had the 7 pin changed on the truck to no avail as that is what the mechanic said was the problem. The mechanic then told me to change the flasher under the dash (10 pin) so I did that and that fixed the trailer issue. Just last week, when I push the brakes on the truck, the signal lights on the mirrors both go at the same time on half solid but do not blink, both signal lights on the dash dimly light up, the third light on the truck signals, and the signal, brake lights and reverse lights outside go nuts. I just noticed today it only does this when the head lights are on, when they are off everything works as it should. Any help would be greatly appreciated.