I couldn't find any writeups on how to add a wire to the existing harness from the body to the tailgate, so I'm going to post how I was able to do this and hopefully save some headache for someone else in the future.

Instructions are for a 2008 H2 SUV.

Disconnect Harness Inside the Vehicle
Steps 2 & 3 may not be required if you're flexible enough, but I needed to do it to get enough room for my hands to get to the connector inside the body.

1. Remove the top trim panel inside the tailgate on the roof
2. Remove the trim panel on rear wall, passenger side. There is a single phillips head screw behind a square cover above the speaker
3. Remove 2 large push clips holding the headliner up
4. Locate where the harness coming from the tailgate comes through the body. Inside the vehicle, stick your hand up above the headliner. At this harness is an electrical connector with a clip holding it on the top. Press the clip with a finger, and use a flat-head screw driver to push/pry the connector loose.
5. The connector has 2 plastic clips that hold it snug into the body. You can see one of them through a hole in the sheet metal looking up - press that in, and an identical one on top. Press that, and the whole connector will pop out of the vehicle.
6. The windshield washer hose does NOT disconnect, so don't yank too hard on this connector.

Disconnect Harness From Liftgate

1. This one is fairly easy. There are 2 plastic clips holding the entire assembly into the tailgate. Use a screwdriver to push and pry until it comes loose.
2. This is not a connector. It is just the plastic & rubber boot that the wires pass through.

To fish my new wire through the rubber boot, I tried a variety of objects, but most were too stiff and wanted to tear the boot, or too soft and couldn't pass through. In the end what worked best for me was using the radio antenna off the truck. I had already removed it and it just laid in the cargo area, so easy peasy. The ball on top of the antenna also helped it to not tear the boot.

Looking at the harness that goes into the body of the truck, there is very little room to add new wires. You do have some space around the washer hose though - on top and bottom are 2 plastic pieces that press against the hose to hold it in place. I was able to shove a screwdriver through, tape a wire to the screwdriver, and wiggle it back through.

 

my amp steps came with a piece of white plastic tube like for water or like the air suspension uses which made it simpler to wire through the rubber accordion boot grommets at the doors. you fish it through the rubber to the otherside push your wire through it then pull the hose out the free end. pretty simple.