My 75 never had lamps until I installed them. The wiring is easy, like BD said, you can use the designated place from the wiring diagram (preferred) or choose any constant hot if you want the lights to come on when the doors are opened. If you want the lights to come on with the light switch too, things are a little more involved. Not hard by any means. I am lazy though so I installed the mentioned toggle switch to the left of the column that could be reached when the door was opened or closed. I am no electrician but that circuit is relatively easy no matter how it's done. Hot wire to whatever switches are desired, wire from switches to bulbs, wire from bulbs to ground.
I just consulted my original (and very fragile) 1975 wiring book and it shows one courtesy lamp located near the left middle of the truck dash. The door switches were optional and were single wire grounding switches (I didn't think I was remembering that wrong), the hot wire (18 OR-40B; meaning 18AWG, orange, circuit 40 'B' meaning after a splice or load at/from the wires origin -the fuse-) comes from the behind the fuse panel on the end closest to the hazard flasher. This 18 OR-40B wire terminates somehow in a splice called splice 40 (also an orange wire), one wire from this splice goes to the light switch terminus 40C and the other goes to straight to the courtesy lamp (40D). That is one half of the circuit, the control half is a little more complicated since it has a few options that could, or could not, be present. Options like a cargo lamp, door switches, or a dome lamp. Those are all options BTW, not necessarily a given. The wire leaving the courtesy lamp is a white wire (18W-156) and it goes directly to the light switch terminus 156 (go figure). Inside the switch this is a simple grounding switch. If that is all that is required that is all there is. BUT, if there is a dome lamp, a cargo lamp, or door switches required/wanted then it goes like this...physically I don't remember what it looks like but the wires are thus
At the courtesy lamp terminal there will be three white wires at or near it, one is previously mentioned and the other two of these (18W-156B and 18W-156C) go to the door switches. Now if a dome lamp is wanted a wire (18W-156D) is spliced into one of these wires and run to the dome lamp. The other side of the dome lamp has a wire (18OR-140) which goes back to the back side of the Bat Fused terminal on the fuse box. These two terminals are fused through the accessory fuse next to them (20A). What this does is backfeed through the courtesy lamp back to the light switch which will control these lamps in addition to the door switches. Clear as iced tea?