I ran one of these on my A3 1.9tdi for 135,000 miles with no problems whatsoever, they're good boxes.
Fitting is easy. Unplug the connector to your cylinder head wiring loom, plug that in to one connector on the box, and the other box connector to your engine where you took the first one from. You can't go wrong, as it's impossible to connect them up wrong! Then it's just a case of connecting the two thin wires to your battery +&- terminals. I used ring terminal connectors at the smaller bolted connections you'll find near the main terminals that your car electrics use.
Find somewhere suitable to locate the box, and away you go!