Auto or stick? If auto, try wiggling the shifter a bit while holding the key in the start position because the neutral start switch may be out of adjustment or the shifter linkage worn out. If stick, you may have a problem with the clutch switch. Slip it off the clutch master cylinder pushrod and jumper the 2 red & blue wire terminals together then try starting it (make sure you're in N with the clutch depressed.)
I'm pretty sure Ford has used the same color wiring for quite a long time, so basically what you'll have to do is trace the red & blue wire from the starter relay back to the ignition switch, checking for power at each connection.
Check my lists of common part numbers in my photos! Updated often!
1991 F350 Lariat XLT, 7.3l, 5-speed, crew cab, SRW, 3.55, 2WD.
1990 Jeep Wrangler Sahara, in the process of a major overhaul/upgrade. OM617 on the horizon! Front & rear Dana 44s from a Wagoneer till I get some 1-ton axles built & narrowed, and eventually a front shackle reversal. Diesel YJ, bay-BEE, and not a cookie-cutter 4BT conversion!