Hi Dirk,
Firstly, I hope when you put it back together it works and will save you purchasing a new unit - water ingress in the rear quarter certainly is not uncommon! if however you are not so lucky I will keep you in mind when I come to sell my mk 3 (which yes, is in good working order.)
although as you can see I have had no replies on here lol and I haven't seen any particularly good deals on eBay as of yet, so until then I won't be parting with the current nav! Keep me posted though with how you get on, fairly often the corrosion repair is enough to get the system workin again. If datatek hasn't already done so, and your system does work again, get yourself a can of darathene and spray all of the electrical connections and circuitry with it - it acts as a sealant against moisture and so even submerged in water your electrical connections would still work correctly without issue.
Ollie