wiring problems with turn signals and running lights,.

I have a 1998 GMC Sonoma pickup that has been sitting for 1 1/2 years. left turn signal blinks rapidly while right TS is ok. If I put on running lights, both turn signals blink rapidly. and the taillights blink strange and then stop all together. The license plate lights were also blinking, I removed the bulbs and taillights still the same. inspected under the rear of the truck and found nothing disturbed. Changed electronic flasher and no changes. If running lights are off, right turn signals operate normally, but left side blinks rapidly. Suspect a ground problem, but ground on rear of frame look good and connected.