This topic is also covered in another thread here on the forum.
I did this about a year ago. I also realized that there is so little space under the bumper AND I also realized my big fat hand had no room to splice into the coolant supply lines where they suggested up inside the engine compartment. What I chose to do, since I have a factory rear heater/AC unit installed was to tap the heater hose that runs to the rear heater unit. The factory hose goes to a valve unit a little aft of the fuel pump. I tapped the supply hose to that valve to feed the filter and ran the return up to the catch tank and tapped in there for the return. There are tons of holes in the frame rails to mount the filter bracket to and access is super easy. I used a Sinister Diesel Universal coolant filter kit and just added some extra hose to make the return line back to the engine compartment. Super easy, all hose connections are very accessible and no drilling of any new holes was required to mount the unit, the frame has a zillion holes in it from the factory down there.
If you have a factory rear heater/AC unit have a look at what I am talking about, you will be glad you did.