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.

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1 answer 1 comment
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.
No, problem has been fixed. It was the flasher but I picked one up at an auto wrecker myself and cost me $10 Cash.