I have been having a problem with the front directionals on my 69 SS. Specifically, my directionals and 4-ways work when the head lights and parking light are not on, when turned on the front directionals and 4-ways do not work they stay on constantly - no breaks. The rear works as it should. I have replaced the flasher, lamps in both units, and the head light switch. Using a test light I have found that from the harness GM connector, where the parking light assemblies plug into works fine on both sides. So I am thinking a ground with in the parking light assemblies themselves???

Any thoughts???

You hit the nail on the head, I would agree that there is a ground problem, Most light problems are a poor ground, Check for tight and clean connections.

Another place to look is the side marker light circuit, Do they both work, Try popping the bulbs out of the side markers and try the signals and lights then, if the signals and lights work fine then look at this harness. I think both wires are hot, one for the signal and the other for the lights. The wiring diagram that I have does not show the side marker circuit.