Well my common sense says that if you have a hole in the spare wheel well, water & muck from the road will get sprayed inside as you drive. I've never noticed a hole there, nor any dampness etc., and i've got a lot of other stuff besides the spare wheel there as well so i go there every once and now. I'll take a look today (if i remember!), but in the meantime can you post a photo so we know exactly where and what you're meaning? I don't see a reason why there should be a hole there.