I have not worked with a VW van, but there are generally only two choices:
- the manufacturer provides a plug-in point under the dash (that's factory pre-wiring, which seems unlikely in this case)
- the brake signal wire to the controller (normally red) is tapped into the brake light switch wiring
So, guessing that the desire is to get to the brake light
switch wiring, that is usually done right at the switch, and the switch is normally mounted under the dash right on the brake pedal lever (or adjacent to it) so that pushing the pedal down triggers the switch. The biggest difficulty would normally be to figure out which wire is the right one...
Given what I have seen on an earlier VW, I would not be surprised if the Eurovan has some bizarre brake light
switch location - odd configurations seems to be the VW way.