Mine has had similar problems in the past, and its usually the turn signal bulbs in the front or rear that are making a bad connection.
The rear bulbs are dual function (marker, and turn/stop), and you can get some funny cross-connects and bad grounding if they are inserted wrong or don't make good contact. Cleaning contacts and making sure the bulb is oriented right usually fixes the problem.
The only in-cab ground I've had problems with is the black wire that grounds the instrument cluster to a post on the parking brake mechanism.
Bruce