Well the bane of the VW's are grounds, you have to have a good Battery to Frame, then Frame to engine ground, and if you haven't replaced the main two grounds it wouldn't do anything but help you to replace them now.
The rear lights were always cascaded from left to right or right to left on the Rabbits and Cabriolets that I have owned. When I have had a flaky issue on my Diesel Rabbit, or my Cabriolets, the first thing that I have done was to separate the grounds left from right, and I have made new grounds on the left to the frame and the right to the frame as it is less resistance of the ground and less flakiness to deal with.
As mentioned I would separate the grounds and tie them to each side respectively as long as you have a good Battery to Frame ground.
If you have good frame points and a good frame connection on the front, then clean and buff an area, Drill a small pilot hole or use a bolt that is grounded but be sure to clean the bolt shiny, as well as the point of contact, and it wouldn't hurt to add a small modicum of Dielectric Grease to the bolt, Never-Seize works as well.
Yes you could run a new wire, but there isn't a need, but alas on the Caddy, the bed had rubber mounts, and they did run a bonding wire from the bolts to the frame IIRC to make sure that the bed was grounded to the frame and it could be that your bonding wires are corroded to the point where they aren't of any use.
So be sure that you ground the Bed to the Frame which is what I would do.