HELP lighting issues

My 98 gmc c2500 vin F is having some issues with brake/turn lamps. Both front turn signals work, and both rears work. When signaling right the lights flash fast like I've got a bad bulb. They flash normal when I turn hazards. Left brake light does not work but right does. Also right front headlight works intermittently.

I have changed all bulbs and both tail light circuit boards. I swapped known good brakelight switch with no change. I have checked connectors and applied dielectric grease to bulbs and connector contacts. I know both circuit boards are good I swapped sides and the problem did not change. I feel like it may be the turn signal/multifunction switch but hate to spend almost $200 on it to find out. Does anyone have any tips or suggestions as to what the heck I should do at this point? Any input is appreciated.


Will

Well, since you are in a wet climate, I would be checking all of my ground connections. Just costs a little time not hard dollars.

I did go through and clean/check all grounds. The tail lights all work off the one ground on the driver side frame rail behind the rear bumper correct? Seems like if any tail light was working that would mean the tail lights have connection with ground.