Ok, good info. The front marker lamps working means that the fuse and the main headlamp switch are good to go.
Dug up the EVTM page for the 1997 Exterior Lamps :
This means to C158 in the driver's kick panel is good, and splice S115 is good ( this is where the circuit branches to the rear of the truck.
I guess I should have asked, are the brake and reverse lights working at full wattage ( i.e. not dimmer then normal ) ?
A quick test is to check the bulb connector in the rear tail light for +12v when the brakes are on with the red lead on Light Green w/ Orange Stripe and the black lead on the black.
If this is +12 V DC, then next is to turn on the parking lamps, and test with the red lead on the brown wire, and the black lead on the black ( same as above ).
While you are at it, check for a brake light at the 4 pin trailer tow adapter, as well as the parking lamps, if / when you move on to step 2 above.
You could also eye ball the rear tail lights to the CHMSL, but this is somewhat subjective, and may not give accurate results.
I am looking for a missing ground, and maybe the brake lamps are using the parkign lamps for a derived ground ???
If not, we are looking between the driver's side kick panel, and the rear of the truck.