Rogier - which elements of the Reiff system are you intending to install? If you are going to install both cylinder base heaters and oil sump heaters I have to believe the installation is a little more complex. When I installed ours recently (Turbo XP system plus oil cooler heater) I opted to put the 115V plug clamped to the #2 cylinder push rod tube, facing forward. This allows me to access the plug through the left cowl inlet. My extension cord then comes out the cowl inlet and along the outside of the lower cowl. I have a Thinsulate engine cowl cover made by Aero Covers - the extension cord runs between the cowl cover and the cowl and exits in an aft direction. This has worked well so far.
I should also mention that I'm very pleased with the Turbo XP system - the day of our first flight saw temperatures starting out at -22C, then warming up to -15C (my personal minimum set for the first flight). I had plugged the Reiff system in the night prior and when I started the engine the oil was at about 105F.