Time-consuming and fiddly, but not difficult.

Provided the space is on your console to take the later switch it is even easier. If it isn't, you'll need to also visit Maplins or similar to get an instantaneous Push-To-Make switch.

You just need to go to the scrappy and get the solenoid from the boot (+ the switch if you are using that), then run a wire from ignition to switch so it is only live with engine off, and then from switch to tailgate where the solenoid needs screwed in and attached to the boot latch.

If you are fitting an aftermarket switch, remember to put it somewhere hidden but that you can reach easily.