HELP: Electrical gremlins reeking havoc on both my beetles

On my 59:

Plugged the two wires into the master cylinder and the passenger side brake light gives me parkers (but nothing on the drivers) then when I brake the passenger one goes out and the drivers side brake light illuminates but nothing on the passenger side.

On my 72:

Same sort of thing. I've done a 60's guard conversion and running snowflake lights and little indicators on the rear. Cannot get any of it to work together: one illuminates whether it's Parker or brake and the other goes out.

It's driving me crazy and I cannot figure out what's going on.
Any help or advice appreciated

Check that you have a good earth return

Sounds like an earthing problem to me.