turn signals

[peep333]

Turn signals work fine when headlights or running lights are off, but will not work when lights are on.

[spider2081]

Are all the light bulb ad their sockets clean?
have you cleaned the grounding pods on the front fenders under the hood?
Have you checked the ground for the tail lights
Do the turn signals work with the engine running and headlights on?
As signal flashers age they become voltage sensitive. Do you have battery voltage at the signal light flasher??

[dinghyguy]

headlight have parking light on as well, so perhaps headlights are not the issue.
looking at the circuit diagram shows the taillight boards are one of the spots where all the systems are together. If the grounds are good, then my next suggestion is to either replace the boards for the taillights BIG$ or perhaps borrow some from someone. failing that i would do as i did and rebuild the boards by drilling out the rivets holding the bulb holder to the boards, cleaning everything with a wire brush on a Dremel (lots of corrosion under the bulb holders) and then soldering or bolting everything back together. I believe there are some posts with more detail. I did that and no more taillight weirdness.
cheers and good luck
dinghyguy

Oh and add relays for the headlights if you have not done that yet.