Turn signal problem

Having a problem with my1966 Mustang Convertible turn signals. When I turn the lever for a right turn both my right and left taillights blink but when I turn the lever for a left turn only the left taillight blinks the correct way. I've tried everything I know which is not much in electrical matters. Would appreciate some advice and help to correct this problem. Thank you!!

turn signal switch in steeringcolumm , my guess.

Bad grounds as well? Pull one bulb at a time, front and rear, and see what corrects the problem. That position with the bulb removed has the problem.