I don't know how it "should" be done, but I brought the power line from the car to the 1ST position on the PD converter and I installed a 20 Amp circuit breaker fuse replacement in that position.
My assumption was that the source with the highest voltage would supply the Buss.
If the PD is not powered (Under tow) then the car's power would pass through and be protected by this circuit breaker.
The car end has a relay to only supply power with the key on and it also has a circuit breaker in the line to protect the wiring.
The first positions are higher current rated then the last ones.
I don't know about the "Mighty Mini" but I did this with my PD4045 converter.