Electrics Rear light - wiring issue?

Hello,

On our RHD 72 Beetle the rear lights are playing up.

With the lights switched on only drivers side illuminates, but when brake pedal is pushed, the drivers side goes out completely and the pax side comes on.

Without lights on, when the brake is applied, pax side is bright, drivers side dim.

Any ideas what might be the cause?

Thanks

Kevin