When I installed my PD4045 I had a "clean" installation with no existing wiring.
I installed a terminal strip for the + and a ground bar for the - behind the unit.
Here is a picture of the mess I made to terminate both the power and grounds.
It is just as important to have a good ground as a good + power connection.
I chose to terminate the cable from the 7 pin TV connector at the terminal strip on the right and did it in pin order to make it easier to troubleshoot. I connected the brakes
and power (ground too) here.
I chose to run the power from the TV through the 12 volt buss and connect it to the terminal strip in the left. The # 1 fuse position in the 12 volt buss has a 20 amp plug in circuit breaker that goes to terminal #1 behind and a jumper to the trailer strip on the right.
With the long run to the TV battery
don't expect the TV to provide a lot of help keeping the battery charged.
If you look closely you might see the ground jumper to the cable that goes to the battery. Later I installed a power measuring shunt to get total amps, volts and power for the complete system.
I wired the trailer with two conductor cables to each consumer of 12 volt power for future troubleshooting and circuit integrity. Since there is no terminal strip on the PD you will need to either install a terminal strip or crimp then in the wires directly.
If you do either be sure to number them and it is a good idea to label than on the front while you remember.
By the way the wheel well behind is steel and welded into the frame on my Scamp
so it is a pretty good ground where the stock Scamp
has a fiberglass wheel well and it is insulated from ground. The lack of easily obtained grounds is one of the big long term problems with single wire power to anywhere.