I recently installed a tow plate on my Mini Cooper S (manual) to pull behind my Sprinter Based ERA class B. We've been towing various vehicles 4 down behind our class A's (3 DP'ers) since 1995. I decided it was time to put toad brakes into the system and chose the Blue Ox Patriot on the floor system. The Blue Ox does use the battery in the Mini for it's power.
I purchased the TOAD-CHARGE kit for just under a $100 online. This kit has absolutely everything you will need to install it and is very well engineered and user friendly. It took about an hour to install into the ERA and the Mini and is totally independent of the Mini and/or ERA wiring. It works great, only draws from the ERA battery when it's running, and regulates the charging of the Mini battery to eliminate overcharging. The system can be moved from toad to toad and/or motorhome with just a little of effort and time.
I HIGHLY recommend this product. And it's great to know that the mini WILL start every time I unhook it when we reach our destination.