Camera's were not allowed in the factories, for industry security reasons. For booking Mercedes factory, I just called the phone number of the factory in Singelfingen. Try this site to find the number:
For the Porsche reservations, you will need to contact your local Porsche dealer, and after they do about 3 faxes, between you, the dealer and the zuffenhausen factory, you will be all set. Actually, that is not too bad, just takes about 1 week to do all the paperwork, versus, Mercedes which was just a phone call.