My lighting wiring is a bit non-standard, so my own truck is not a good reference.
I've attached a light switch operation diagram (shows the various switch positions) and an internal wiring diagram for the 3 lever light switch. Tracing the connections here helps figure out how things are supposed to work.
In the switch internal wiring diagram:
15 = Power feed to the switch
B = Instrument Panel Lights (wire #40)
16 = Headlights (via the floor mounted dimmer switch)
21 = Tail Lights (rear of truck)
22 = Stop Light (a.k.a. Brake Light)
75 - (Pin A) power feed to Stop Light Switch
75 = (pin K) return from Stop Light Switch
491 = Parking Lights (front of truck)
As I read the diagram, the only way to get the Parking Lights (on the front of the truck) ON is to have the lower left switch in "PARK",
and the main switch in either "STOP LIGHT" or "SERVICE DRIVE". No power is fed to the lower left switch when the upper switch is in the OFF position, so no parking lights when the upper switch is off, regardless of what the lower left switch is set for. However, switching the Parking lights on the front of the truck ON turns the Headlights off. With the stock wiring, there is no way to have both of the headlights and the parking lights on at once.
Tail lights (wire 21) are energized ONLY if upper switch is in Service Drive position
So with standard wiring, for normal driving:
- For daytime driving, the main (upper) switch should be in STOP LIGHT to enable the brake light function. (lower left switch can be in off or park, depending on whether you want the parking lights in front on or not.)
- For night time driving, the upper switch should be in SERVICE DRIVE to turn on the headlights and tail lights (this position also enables the brake light function)
If I've messed up my tracing, hopefully someone will come along and correct my interpretation.
NOTE: Edited from original post. I had confused Parking Lights (which on the M37 are in front only) with the tail lights.