Quite a while ago I started to see that the area around the side door is wet and the paint starts to peel off. Time to go for cause.
The outside light
Didn’t start so obvious… I found the wood that was used to fix the window curtains to be wet at first. When I followed the water trace I realized that the wood to cover the door frame was also wet. After quite some back and forth with my thoughts if should really start opening the walls, I decided to go for it. And what I found was a leaky outside light from above the side door. It was old and cracked and water was coming inside from the hole that was used to guide the wire from inside to the lamp.
I bought a new light, removed the old one, cleaned up the surface and glued it with some good old Sikaflex to the roof. And I made sure that there was enough Sikaflex sealing going into the hole that there could not come water into the van anymore. Side note for now: I did not yet connect the wires to the electric system of the van as I did not have appropriate tools at hand.
Poorly sealed holes in the roof
When it finally, weeks later, was time to connect the wires of my new outside light, I realized that the inside was still wet in that area. So I had to dig a bit deeper – or, actually, higher. I needed to open more of the inside wall covers to discover that there were two old screw holes that were poorly sealed. Probably at some point there was an outside tent attached or something similar. The holes were not big (maybe 5mm in diameter) and difficult to see from outside. But apparently the sealing that was used to fill the hole had partly lost its connection to the roof and let water constantly flow into the inside and under the PU-foam insulation. It kept running down the wall and used parts of the metal frame of the van (vertically and horizontally) to nicely distribute within the wall. And depending on wether to bus would be tilted to the front or the back when parking, it would run to the front of the van or to the back covering a quite big area with water.
After all I ended up removing quite a big area of the right wall including the back of the bench because the water had spread so far. Thanks to the horizontal metal bar from the original VW LT roof, it seems to have gone about 1,5m far to the back of the VAN and totally soaked the wooden wall cover there too. For safety reasons I still have to check whether there is another hole in the back of the van.