abster641, after looking more closely at the kit contents I do NOT recommend the 66 21 0 303 083 ADDITIONAL KIT FOR CAMERA. It does NOT include the reversing camera relay and it's wire harness. As you plan to use an
aftermarket camera... I would get the more expensive 66-21-0-397-639 REAR-VIEW CAMERA & SWITCHBOX.
This kit includes almost all the parts from the 0-303-083 and you do get the relay and its harness... which is needed to run up into the tailgate and down to your liscense plate area.
I'm assuming you are mounting the aftermarket camera in some custom location? It would difficult to use an aftermarket camera with the "plate and switch that replaces the boot switch on the E39 and E46" which KiwiJochen mentions below. You do NOT get this plate w/ the 0 397 639 kit.
For a good example using the 0-397-639 kit check out this write-up from
www.e38.org:
http://www.bimmerboard.com/forums/posts/516841