jlf: you don't say what 5er you have. A number of the FR brands have started coming with the pre-wired, pre-installed mount for the Furrion backup camera. That's about $400 ordered through FR parts and would take about 5 minutes to install.
Others have had good luck with the Voyager mentioned previously and lots of other eBay deals.
Myself, I needed a new GPS anyway so bought the Garmin RV760LMT with the BC-20 backup camera. The Garmin camera installed in the Furrion mount with few modifications but took me about 2 hours.
Use the search box above and search on 'wireless camera' or 'backup camera' and you'll find my report plus hours more reading - its a frequent topic.
2015 Rockwood Signature UltraLite 8282WS Platinum, GY Marathon LRD, TST 507RV TPMS
2005 GMC 2500HD CCSB D/A, Curt E16, Prodigy P2, Garmin RV760LMT w/BC-20 b/u cam