Me again!

I forgot to add that one fairly simple way to check is to pull out the little
switch in the droor frame which works the interior light.  If the wire
running to it is BROWN then your bus IS meant to be neg to earth (ie, brown =
earth, and opening the door 'closes' the earth).  If the wire is RED on the
other hand (not likely I think) then your bus is configured for positive to
earth.........

Rob K.