I have a 68 tach that does not work. I am trying to find out is there any common problem with them. Or who to send it to?

Possible the ground wire to it maybe bad.Or have the connector plugged in backward,where the feed is going to the ground side.If can remember right, is L shaped,2 female connector.Would be plugged in with the L faced other way.Way to check if have good ground inside,or where the ground connection is,is using a multimeter.Connect on side to that post and touch other end to housing.One will give off a resistance signal.If don't get on on either post might be bad connection somewhere or oxidation or dirt not making it have a good ground.