[shorod]

My first thought is that you have a grounding issue for the turn signals somewhere which could explain why the instrument cluster lights flash - the turn signals are getting their ground through those bulbs. The first place I'd check in this type of scenario would be for aftermarket trailer wiring, and if installed, confirm whether or not all connections were made with solder and adhesive lined shrink sleeving. If not, there's a good chance corrosion has set in and several feet of wiring will need to be replaced in order to get good, solderable wire.

You don't mention what year your Silverado is so I'm not able to look up wiring diagrams to see if there is a logical explanation for the left side to act up but not the right side.

-Rod