Right turn signal stops when lights turned on

[spider2081]

Both filaments glowing when right signal is selected usually is a poor ground. Your grounds for the signal light are the right fender ground pod and associated wires. The ground pod near the flasher under the dash. The ground wire in the trunk, originally under the left tail light mounting screw.

Do you have a volt meter or test light???
Do you have the wire diagram for your car??
Where are you located??
Have you checked the harness that connects the rear tail light assemblies together. The right side tail light PC board makes all its connections through the left PC board. This includes the ground. You mentioned changing PC boards so they are most likely good.
The inter connecting harness/connections could cause this problem.

Are there any other electrical symptoms you have not mentioned??

The next test I would make is at C18 the connector for the signal light switch at base of steering post. The light blue wire in the connector,(right signal out of the switch) needs to be pulsing with the flasher full battery voltage with the headlights off and on.

[Christo]

I am waiting for a rainy day and then I will take the rear harness out of the car. The car was bumped in the back 15 years ago. The area where the license plate goes was pushed in and had to be pulled out. I noticed the license plate lights don't light up and the right reverse light blows each time I put a bulb in. I suspect a nick in the harness now. I will report back once I have pulled it out.