I have 2011 Sierra SLE (?) Ext. Cab 4x4. the left side signal started rapid firing. When I checked the bulbs I noticed that besides flashing faster than normal that the bulbs were dim & the back up light was also flashing. i'm thinking that there must be a short somewhere in the harness, possibly in the trailer light loom. anyone seen this before or want to hazard a guess as to where to start looking??