Found the same symptom on a GMC 1500 pick-up with the signals in the mirrors. When they dim or intermittently work......check the ground. We found with the use of a VOM that the incomming voltage was correct and consistent when measured to a known ground source. When testing the harness ground we found reduced voltage. Thus, a bad ground. On the GMC they grounded the mirror wiring to a single ground inside the door panel. Salt and corrosion left that ground source bad.
Generally, the motors for the mirror adjust are grounded via wire that goes through the harness that enters the door panel at the hinge. Not always the case on a signal mirror. Not sure if this helps but it may be a starting point.