Where exactly is the best place to mount it?
Install it in as much airflow as possible, near the hottest coolant point possible.
Coolant return hose to the radiator is the hottest near the engine, but least airflow. Further up near the rad is better airflow, but not as hot. Guess it depends on how fast you go.
If you really want to make a difference - go get some 3/4 EMT and an Ideal conduit bender. With a few creative bends, you can make some replacements for the rubber portions of the hose that will shed heat great since the EMT is aluminum.
Cut about 4" off the original hose ends and buy some clamps to make sure it seals tight, and make sure it doesn't contact the frame or any hard parts. The buzzing sounds will drive you nuts.
I've done this with all my quads and the RZR now - the fan on the RZR hardly ever comes on now, where it would come on ever time I came to a stop before.
I have a built Raptor that would overheat and hit the temp light even after a $400 aftermarket radiator, and the tubing trick will let me ride without over heating. The fan only comes on now if I'm at a stop for 5+ minutes, and I've never seen the temp light since. Guestimate it pulled 40+ degrees out of the coolant temps for 3 ft. of conduit, but bending was tricky and took a few tries and scrapped pieces of EMT.
I bring conduit and a bender with me to the dunes now, and I've built hard lines for all my friends quads. Its cheap and makes a world of difference because the whole hose sheds heat, as opposed to a 3" section.