Well, I can safely say that DIY'ing the towbar electrics isn't a job of anyone who's not experienced with auto-electrics!!!
It's take me 3 hours to run the power-feed ( 2 x 15A fused cables) from the battery to the boot area, and then working out which cables to tap in to for the various functions.
Having completed it, I'm glad I did it, as I've learnt a hell of a lot. But if I hadn't had some previous experience, I would not have even attempted it. And the ~£100 I was quoted to get a 'pro' to fit the towbar and electrics, is quite reasonable, as it's about a 4-5hr job I would say.