This just doesn't make sense. My first impression is the headlight switch, but you've replaced it 4 times.....and I just can't imagine how 4 of them could all be bad; with the same problem.
The front marker lights and the cab clearance lights are powered by Fuse 'F' (20A).
This fuse supplies battery power via the Pink/Red wire to connector C1-Pin2 of the light switch.
The light switch connects this power to to the Black/Yellow wire in connector C1-Pin1, either the park or headlight position.
That wire connects to a splice joint that distributes the power on the same wire color to the 2 front marker lights and the 5 cab clearance lights which have separate grounds.
Also from that splice the wire continues to another splice that distributes power to the tail lights and license plate lights.
Based on this, the problem keeps pointing at the switch, as if there was a splice issue, that the lights would not work regardless of the switch position.
Can you check for power at the light switch on the Pink/Red, then on the Black/Yellow that's right beside the Pink/Red. It should be on the Pink/Red all the time, and on teh Black/Yellow in both the headlight and park light switch positions.
This is a strange one!