My 2007 2500 express has suddenly developed a problem with the left turn signal. I replaced the turn signal assembly, Cleaned the circular copper contact ring on the column but the issue still remains. The light will blink sometimes but other times there is nothing I can do to make it blink.

 

You will need a multimeter and a schematic from
https://www.gmupfitter.com/pages/body-builder-manuals
Standard electrical troubleshooting applies to track down the problem.

https://www.aa1car.com/library/tselec.htm

 

There are a lot of places were that failure could happen. I would troubleshoot the bulb and connector first. Ensure all of that is good because it is exposed to the elements and greater/faster temperature swings.

 

The connector at the rear, behind the lens on the inside of the van.