I'm not sure what link I posted before but I did some digging and found this thread:
My son is adding rooftop marker/clearance lights to his ‘79. How was the wiring for these ran? Is the windshield post hollow? Or does it go down the back corner of the cab and then up to the dash?
www.gmsquarebody.com
and looking around in there I found a written explanation that was posted on another site:
As your looking at the roof from the front bumper let's number the lights 1 - 5, passengers side to drivers side.
1. Diameter of all the holes is 1 3/8"
2. Center 3 holes (#'s 2,3 & 4) are located 5 3/16" back from the edge of windshield gasket to center of hole. Front leading edge of 3 center holes is 4 1/2" from edge of windshield gasket.
3. Center 3 holes measure 6 3/8" center to center to center. 12 3/4" center to center from hole 2 to hole 4.
4. Holes 1 & 5 (outer most 2 holes) are located 5 3/16" from the inside edge of the drip rail to the center of the hole with the edge of hole measuring @ 4 1/2" from the inside edge of the drip rail.
5. Holes 1 & 5 measure 6 3/16" from the edge of the windshield gasket to the center of the holes with the edge of the holes measuring @ 5" from gasket edge.
6. Holes 1 & 2 measure center to center 17 3/8" with 16" from edge of hole to edge of hole.
Holes 4 & 5 measure the same as 1 & 2.
7 Screw mounting holes are 2 3/4" across the center of the larger holes.
8. Outer 2 lights are angled outwards about 10 to 15 degrees from straight forward.
