1997 GMC Sierra extended cab. Left turn signal not working, I have had this problem for a few days now. Right signal works fine, left will flash on the dash indicator just once then dim same as all the exterior lights. Had it in my shop last night changing a right tail light board and then it worked for a few hours then back to same problem. Tried putting a new tail light board in, tried a new flasher as well tried new signal switch still having the problem consistantly. All the lights will light up but no flashing. Thinking it may be ground but hoping someone has had some problem or has any suggestions. Thanks, Steve