Hi Stephen, i hope you have sorted your reverse light bulb out by now but i can't see any replies to your question so here goes.
lift your tail gate and look for the plastic pop outs in the rain channel pop them out till you see the nuts ( 10mm ) there are 2 of them one holding the top half of the light in and one for the bottom.
remove both, you need the bottom cluster out for reverse but the top cluster overlaps it. then put your finger inside one of the holes you just made and pull, if it's any thing like mine it will be very tight to remove but the light has one nut and two push in pull out type studs holding each of the clusters in place. hope this helps. ALV, Wakefield
ALV, January 2008