When using right turn signal, all four blinkers flash on 2004 GMC Sierra 2500 HD

I checked all related fuses and disconnected the rear main harness & trailer plug connector and all appears fine, no corrosion. Even with the rear harness connector and the trailer connectors both disconnected, I tried the RH turn signal and all four blinkers flash including the mirror signals in my factory extendable mirrors. Any idea what I should check next. Oh, one more thing, it used to be an intermitant problem when it was rainy/humid and cold conditions but recently it dry outside and the issue has been going on since Friday Sept 24/10.

by in Pembina, ND on September 28, 2010
1 answer 1 comment
ANSWER by on January 16, 2012
Do you still have this problem? It possibly the flasher relay. Its located on the left side behind the dash on the driverside. Its a pain to get your hand on and the part is about $75 from the dealer.
COMMENT by on January 17, 2012
No, problem has been fixed. It was the flasher but I picked one up at an auto wrecker myself and cost me $10 Cash.