The hardest part by far is fitting it in to the bootlid strip, This is very fiddly and takes a lot of time. The complete kit barely fits in to the strip and I would say this retrofit is the least "plug and play" I have had to do.
The wiring is the easy part and so long as you follow the instructions you will be fine.
Another difficult aspect for me (E46) was that you had to enlarge the hole in the bootlid where the loom comes through in to the back of the strip handle.

Here are some shots: