Factory-original 1966 and 1967 Broncos will have the warranty plate with VIN number on the back of the glove box door. Models from 1968-1977 will have it on the driver kick plate panel. VIN numbers are also be stamped on the passenger-side frame below and rear of the starter (which can be seen by looking down into the engine compartment).
NOTE: in the 60s and 70s, VIN numbers were hand-stamped by Ford employees and it was not uncommon for mistakes to be made. If your VIN number is not returning the correct results, try alternatives such as replacing a “1” with letter “I” or “0” with letter “O”, etc.
Sample VIN Plate Images
1966-1967
1968-1977