Hooking up the auto-dimming mirror is not that difficult. If you have the OHC already, you already have most (if not all) of what you need in the OHC wiring harness.
If it is ONLY an auto-dimming mirror, and does not have the auto-headlights feature (or, it has it, but you don't want to hook it up), there are 3 wires you need to be concerned about
+12V constant (Green/Yellow) - this connects to the Dome Light Power in the OHC (Black/Blue)
+12V switched (Red/Yellow) - this connects to the Console Electronics Power in the OHC (Gray/Yellow)
Ground (Black) - this connects to the Ground in the OHC (Black)
- there is one more wire, a (Black/Pink) one -- this is the Reverse Sense wire. You can run another wire from the mirror to the (Black/Pink) wire in the bundle of wires that drops from the bottom of the dash to the transmission tunnel (just to the left of the center of the dash). If you connect this wire, the auto-dim feature will be disabled when your truck is in reverse. A lot of folks don't bother hooking this one up, but it's there if you want to do it.
If you have the auto-dim mirror with the auto-headlights option, then there is one additional wire (White/Purple) that you need to run down to the vicinity of your headlight switch. This wire is a trigger that turns your headlights and parking lights on/off automatically. It becomes the "trigger" for two relays [one connected in parallel with your headlight circuit, and one connected in parallel with your parking light circuit]. The "auto-dim / auto-headlights" writeup in the how-to section details how to wire up the two relays needed for this function.
EDIT: actually, in reading back through that and thinking about it, the (Green/Yellow) [+12V constant] is probably only used for the auto-headlights feature as well -- providing power to the circuit after ignition-off, so that it can keep your headlights/parking lights on for <> number of seconds after the truck has been shut off. The auto-dimming function of the mirror is pretty much self-contained and only requires switched 12V (accessory power) and ground to operate.