I run a wire from the motor home battery - fuse - 7 pole rv plug - fuse - car battery. The car battery must be disconnected from the car electrical systems. I use this wire to keep the battery charged and to power my brake buddy. The car battery also acts as backup power to the brake buddy incase the car becomes separated from the motor home.
Two things I am thinking about changing. Adding a diode to the wire and changing the power plug type on the brake buddy.
I have yet to find a decent place to mount the 7 pole rv connector on the focus.
12000 miles towing the focus with no problems.
1999 Bounder 32H Ford V10, 2012 Ford Focus, Pretty DW, 2 cats. Retired USAF