I guess as the wires in the rear glass age the connection for the high brake light isn't always making a good connection ?

I cleaned up everything in the socket and the wire connections on the glass and put in a new bulb and it works sometimes - especially if you give the door a good close but then sometimes it stops

Any good solutions to get a good connection ?

 

Check for broken wires in the harness running from the body to the door. From opening and closing the door they can fail.