I would imagine it's the one on the nearside that operates the light.  My light hasn't worked since I bought the Bongo, but I suspect that's a blown bulb.
Some while ago I raised the roof to check it out, and when I lowered it again the beeper wouldn't stop.  That turned out to be the offside microswitch.  As a "temporary" repair (to shut it up!) I disconnected the wires to the microswitch and joined them together with an insulated joint.  I really will get round to replacing the microswitch one day, honest.  
 
 
Now the bad news.  

  The nearside microswitch is much harder to get at as the wiring is completely hidden.  I couldn't see how to get at that one without removing the roof!  
