I just did a 14 circuit kw on my '79 and still had plenty left over. What are you planning, home theater?
I've been meaning to write up what I did with pics, but here's the cliff notes. Other people May agree with me, some won't. Your call.
I eliminated the bulkhead connectors on the fire wall and patched the holes. Made the firewall a lot cleaner. All of the engine and tail light looms went out the penetration behind the master cylinder. I capped that hole with a 3x5 piece of sheet metal and drilled holes for gromets for the individual looms. All of my wires went through this space.
For the tail lights, and all of the stuff running back, I ditched the factory method, and followed the driver's side frame rail. I anchored my loom with adell clamps drilled right into the frame. All of the splice points happen in the well behind the left tail light.
Getting to your specific question, your '79 has running lights, back-up lights brake lights and turn signals. You'll need the manual to show you which elements do what. One bulb is dedicated to back-up, the other is a dual filament that covers the running lights, turn signal, and brake. Pretty easy to figure out which is which once you start pulling things apart.
One gotcha that the kw harness doesn't address (there are several, but its what I expected) is the dome light near the tailgate. You'll need to run two wires down the tail light loom to get it right. I used the "dome light" wire and the "electric fuel pump" wire. Dome light was positive, fuel pump negative. Instead of grounding the dome light the way the factory did, cut and splice the ground to the fuel pump wire (or if you need the fuel pump wire, run another in the loom for this purpose). The courtesy lights under the dash also need two wires. Follow the kw manual here. Its pretty self explanatory. One wire will be 12v from the fuse box that gets split 3 ways: dome light positive and the two positives of the courtesy lights. Then the negative wires of the lights need to come to a common point. Also a ground wire from the light switch gets tied in here. At this point, you can now tie your door jamb switches to this negative ground cluster. When either switch goes to ground (opening the door), the circuit will complete and you'll have lights.
If you need more help, pm me and I'll help as I can. But its best if you solve it here on the forum so everyone can learn.
Ps-do a search for gauges and you'll see several posts as to what folks have done regarding aftermarket gauges as well as stock ones.
Also, I didn't drill the frame, but rather the pinched seam of the body tub right above it.
