With the Vortex search in shambles now, I figured I would bring a well known mod from the Mk1 community to the Cab side. A rather busy holiday weekend at the junkyard means slim pickings. But I was able to get my hands on an Mk3 dome light (non-sunroof). I also cut a pigtail's worth of the wiring connector for the light.
The Mk3 dome light has additional circuitry that leaves the light on for 5 seconds after the door is closed.
BEFORE STARTING - DISCONNECT BATTERY. You will be messing with a hot wire that can very easily touch ground and ruin a wiring harness from the light to the fuse box and lots of things in between.
It turns out VW stuck to the same wiring scheme between Mk1 and Mk3. Red = hot, Brown/Wht = each of the door switches, Brown = straight ground. Mk1 (your original light) shown below removed from the front window frame. A small flat blade screwdriver pressed on the metal tab on the right side will release the light from the window frame.
Since I am reluctant to cut factory wiring, I chose to splice in the Mk3 as follows:
Take old Mk1 light that has the thin push on connectors and drill a hole to run the Mk3 pigtail through. I soldered it through the hole for a secure connection and then covered with heat-shrink tubing. This takes care of the Brown/Wht and Brown wiring connections to the existing car wires. There will be an extra brown and extra red pigtail on the Mk3 harness - just cut these off.
The red one is the easy one. Just make sure it is covered with heat-shrink tubing so it cannot short to the chassis or other wires.
Reconnect the battery. When you open the door with the switch set to "door" in German-icon-graphic-speak, the light will turn on. When you close the door, the light will stay on for 5-6 seconds and then turn off automatically.