i have some experience, thoughts and insight for you here. i did this mod on my 11 XLT about 3 months ago. i absolutely love having the mod function both for my running, and turn lights. however, i DO NOT recommend having your rear facing LED's illuminate with your parking lights. i could be wrong here, but it may be illegal as well (not trying to be a goody goody two shoes here, but trying to avoid you potential misery).
to avoid this particular setup, i would however recommend doing all of your resistor/diode work inside the mirror housing. it is very easy to do, and you will be much happier doing the work while standing beside the mirror, rather than laying on your back underneath the dash. afterall, the mirrors come apart fairly easily, and will make the job much easier. i know that a lot of members are worried about breaking the mirror glass, and i can understand since they are expensive to replace. however, if you take your time, and really understand how they can be removed, i think you will be fine. (i would not do this outside, unless you are in a warm environment though).
i also understand that running wires through the door jambs can be a pain in the *****. i decided to remove each door from the truck to get better access to the wiring plugs. then i was able to drill a small hole, and run a wire through for the parking light feature. when i was done, you can barely see the wire. i did install connectors, so i can still remove the door in the future if needed.
How i ran the wire through the door jamb plug:
This pic here shows how i fished the wire back into the rubber boot:
And with the door back on, and connector installed:
More to follow.....