Can't tell you where the backup light switch is on the manual (though 
     I suspect that in good VW tradition it is on the side of the gearbox).
     
     Empty socket in tail light housing is for the near-standard equipment 
     European rear FOG light (judging from the rental cars I've used in 
     Germany, it's probably a required feature).  I had my left light apart 
     just last night and was looking at this.  It uses a 32 candlepower 
     bulb (same as the stop light or turn signal).  There's a trace for it 
     on the housing, but no contact for the tip of the bulb; I imagine it 
     would be pretty easy to retrofit the contact, taking it from a "donor" 
     housing.  There's also a empty contact location on the plug and 
     socket, but wiring would have to be installed; is this what the extra 
     (grey w/green stripe wire in the main harness is for?  Probably not.
     
     I believe the right housing also has the same features, but typically 
     only the left location is wired up on the Continent, probably to avoid 
     being confused with brake lights.
     
     This is a real cool feature if used properly.  I once followed a Fiat 
     through a dense Lombardian fog - I was able to keep a safe distance 
     because of the other car's rear fog light - couldn't see his tail 
     lights at all.
     
     BTW, the blank on the instrument panel is for the two-position fog 
     light switch to activate the rear as well as front fog lights (if 
     installed).  But of course you knew that.
     
     -Blaine