ahh i'm in exactly the same boat, mine are smashed to pieces as well so thinking about doing some sort of bodge. I've read on other forums as well as somewhere on here that the way to do it is heat the unit up and thus soften the glue. The one I saw on here mentioned using near boiling water but that seems messy and more dangerous to me. Looking at the xenons they have a few points around the rim where there appears to be a joint and some glue but i'm assuming they are glued all the way round. Basically it's gonna be a case of whack it in the oven at 70, 80, 90, 100 degrees etc until you find the heat which turns the glue soft enought to pry it apart. Obviously you need to be careful not to go so hot you melt the plastic.
the alternative would be to cut a section out of the back of the casing to give access and then glue that back on with araldite/epox, superglue or similar strong adhesive. Need to take another look at mine see if there is a suitable section to cut out.