Easy job.
You will need to run a cable from the front of the dash along to the back.
I did it in my MK3 and ran it in braiding along the plastics near the doors. There is actually a circle "guide" for you to cut the original but GTI cards are just better and fit right in.
Took me about 2 hours I think but that was putting proper connectors in the looms etc and soldering and braiding everything.