VW Electrics bad earths
Behind main fuse box in front of (Right hand drive, passenger side) fusebox. there are 2 "crowns" of earths that may want attention. Count how many spades terminals are free so you don't miss any putting them back. might be an idea with all those earths hanging around to disconnect the earth strap from the battery as the fuse panel is still live. You may depending on the state of the terminals want to replace them, use metal ones, not plastic coated as the space between spades is limited. I held each one in a pair of needle nose pliers and wire brushed them.
no interior lights
no dash lights
no tail or numberplate light
all at once
use the flasher and they all work again as long as you hold it
checked all the fuses on the board ...ok
late fuse board fitted This happened to kevtherev.
He fixed it by removing the fuse board and checking the 'crowns'as the symptoms pointed to earth connection  (haynes) wiring diagram. connection  was common to all of the non functioning items
sure enough the crown was badly earthed...probably due water ingress, or recent fitting of a radio and speaker wires
Footnote...A poor connection of the main feed into the fuse board (thick red wire, white connector) will also cause similar problems
There should be at least one earth going from gearbox to chasssis, some folks add a second just to be sure. Update 2012 Recent findings by Aidan the resident gearbox re-builder suggest that the front nose earth on the gearbox is leading to premature failure of the box due to nose cone corrosion (disimilar metals + electrical connection + small gap for water to gather) , and seems to be particularly an issue with 5 speed boxes, he recommends that the earth strap be connected as close to the starter motor as possible and the nose cone earth is removed particularly on 5 speed boxes where this is more of an issue.
Aidan I advise moving the earth to the starter motor on 5 speed boxes to reduce the electrolytic corrosive effects of sticking a 12V potential across a steel plate that has a 1mm gap to a magalloy case where with the aid of some salty water you can see massive corosion of the gearbox end case; the 4 speeds suffer much less from this though you will often see quite heavy corrosion of the bosses that the gearbox mount studs fix into; using the gearbox as the earth contact was just a production line simplification (engine and gearbox lifted up into engine bay and mount bolted on with earth in one action) and doesn't really do anything for efficient cranking Similarly the permenant live across the reverse switch on the gearbox together with the permenant earth to the gearbox causes corrosion of the case around the reverse switch housing and is why in late 1989 VW added a paper gasket to that mating face to mitigate the effects as by then they would have seen a lot of exchange gearbox cores coming back that weren't suitable for reuse due to the corrosion thus caused, and is why I have always fitted the paper gasket, now obsolete from VW and classic parts but which I make myself, though it has been remanufactured in america (but is a bit pricey)
Engine block to left of engine block to engine bay. Particularly on 2.1 injection engines this is a prone point for failure after you have checked the gearbox one above.
Rear Light cluster earths There is an earthing point on the left hand side of the engine bay in the vertical metal wall, this holds three ring terminals which are the earths for your rear lights. I was measuring 8 volts to my rear lights until I changed this. Its harder to get to on a diesel as the air box is in the way . As soon as I put any pressure on the 10mm nut it sheared off with the rust I made a new connection to the battery negative terminal and the rear lights are noticeably brighter. On a petrol van you may need to find an alternative earth point.