Sorry Rob, no.
Remove the rear lens covers and take the brake / side light bulb out. If it's a combined bulb - the type with 2 filaments - just make sure that the bulb was put in the right way round - it is possible to put them in "upside down"
If you have separate bulbs for the brake and side lights, then it's diagnostic time:
I assume you have the "12N+S" 7 pin plug and sockets on both the van and the car? and not a 13 pin car socket and an adaptor to the van?
The reason I ask is that a number of the 13 pin to 12N+S have been reported as being faulty or incorrectly wired.
So now it's time for a careful inspection of the 12N plug and socket:
1. Download a wiring diagram for the 12N socket from the Internet - if you need a link go to our Eriba site - http://www.eriba-amiga.co.uk/otr_towbar.htm - this has a diagram of the pins and the wire colours
2. If you have a digital voltmeter, then check that 12V appears between pin 6 and 3 on the car socket when a friend presses the brake pedal.
If you don't have a voltmeter - then Maplin, or Tchibo, or Aldi have perfectly good meters for under