First place I would check is the outside coach lights, turn signals etc. for corrosion and possible misplaced bulbs. Once had a semi come into the shop and when he turned the signals on all
the marker lights blinked, had an 1156 in a 1157 socket and it was feeding back, corrosion can do that also. Most likely a ground somewhere. i wouldn't check the ground with a jumper I
would remove it from the chassis, clean and refasten. Like what was said, start with clean secure connections first. Let us know what you find.