Nuevo / Boxer Rear light cluster

I have had the real light unit off my 2003 Nuevo, as the Reversing light and Rear light aren't working. I replaced the bulbs, to no avail. Now I'm thinking this is unlikely to be a fuse issue, as the indicator works fine,  has anybody got any words of wisdom before I find someone to fix it? - wires just disappear into the void, and I have no idea where to start, if it can be fixed by a mere mortal such as myself!!  
/Cheers, in advance!

Can you get to check the earth connection?

That's often the problem when some work but others don't.

I've not done this yet, as had to get home. Am going back on monday and I'll check it first. There is an on-site engineer where we keep the van, so if all else fails i can get him on it, but I'd like to have a fiddle first! There has been water ingress, I should add, the metal chassis of the cluster has oxidised somewhat, so I'll also need to try and re-seal it!

Well, that was simple enough! I'm guessing that the water ingress was down to one of those pesky pressure washers, but it had caused the chassis of the cluster to oxidise. I stripped it all down, dipped all the spade terminals in vinegar, which got them shiny new in about 2 minutes, then used wet & dry to thoroughly clean all the points of contact, and voila, all is good. All very easy, but as a new owner, with little experience in any vehicle maintenance, I was all set to hand it over to a man and pay him to fix it, but with a little encouragement from here and a facebook group for MH maintenance, I did it myself. Very satisfying! Thanks.