1971 Karmann Ghia bulb question

I have 71 Karmann Ghia coupe. I cannot get the back-up bulbs to make contact with the sockets. There is Hot and Ground IN the socket, and the bulbs work when tested. They just don’t make good contact. I’ve bent in the Hot tab, wire-brushed the surfaces, and nothing (one brief glow, once). I’ve tried wrapping a thin wire around the bulb, and that worked for awhile; tried a light application of dialectical grease without success.
Any suggestions to improve that contact? Thank you!

Can you remove the lamp socket? Possibly there is a generic socket you could use.

The tail, brake & backup are all part of one cast aluminium piece. I considered replacing with used set I have, but getting to the right side means pulling air cleaner and oil reservoir. Then I doubt I can get the nuts off! But thanks!

This should have the old style bulbs with the lead domes on each side. What can happen with age and mostly vibration over the years is that the dome will be vibrated away and become concave on the part where it makes contact with the socket.

The bulb is good, but the terminal connection on the bulb is not. Take a look at the bulbs closely and note if you can see a concave or even flat surface on those domes. New bulbs if so. Hope that helps; and I’ve seen this on later model and different model cars than yours. Subaru even had an issue with it once upon a time.