Headlight switch wiring
D = Dome lamp, connects to ground through the metal body of the switch.
I = Instrument, feeds from B2 through the dimmer in the headlight switch through Fuse #1 and then to the dash lights.
H = Headlight, feeds from B1 through the internal circuit breaker out the dimmer switch on the floor.
B2 Power comes from the battery, through Fuse #8 to the headlamp switch, provides power for Front Parking lamps, Rear taillights, and the Instrument dash lights.
R = Rear lights (In 68? and newer cars this also includes the front parking lights & the 4 sidemarkers.) This feeds power from B2 to all of the exterior lights except the headlamps.
P = Parking lights This is not used on 68? and newer cars.
B1 Power comes from the battery (does not go through the fuse box) into the internal circuit breaker in the headlight switch and then to the headlamps.
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Note 1: Fuse #8 is 20amps, this provides power to all the exterior lights (except headlamps), the instrument & other dash lamps, and the horn. The power for the dash lamps also goes through Fuse #1. This is a 3 Amp fuse.
This is a safety feature!
If something shorts out in the dash lights, it should blow the 3 Amp fuse first, and not kill the taillights.
If something shorts out in the taillights, it will blow the 20 Amp fuse. This will kill the dashlights too, so you know there is a problem before you get rearended!
Note 2: The dome lights has + power (pink wire) at all time. It is controlled by the ground connection (yellow wire) by the headlight switch, or the door jamb switches. This can be confusing since it's the opposite of most auto wiring.
Note 3: On earlier cars (pre 68?) the front parking lights went off, when the headlamps were on. The taillights were on in both cases, so this is why the separate P & R connections were needed. They changed the wiring, but didnt bother to redesign the switch.
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This info comes from the '74 FSM, and way too much time under the dash of my cars. ;)
The diagrams at mymopar.com are ok, but not nearly as detailed as the FSM.