GMC Repair: Rear Blinker, turn signal switch, chevy truck

Question
I have a 96 Yukon and my left rear blinker wont work.  I tried replacing it with another assembly from a 95 Chevy Truck, but it still wont work and the assembly from my yukon works on the truck.  All other blinkers work but that one. Could it be a problem with the wiring?  Please help!

Answer
Hi
A Few questions
Does the left front work? Does it do anything such as do they blink very fast or nothing at all. are they dim? Do you have any type of trailer wiring done on vehicle? If it is ONLY the left rear, and if there is NO other aftermarket wiring done (trailer) I would have to guess the Multi function (turn Signal) switch. Before replacing this, though I would take it to a repair shop your comfortable with and get them to check and see if the yellow wire (ckt.18)on the left taillamp circuit is dead. If so they can check the wire and if all is good it almost has to be the switch.
Just a suggestion from experience.
Good Luck
Paul