well, starting , at the beginning...
Confirm the battery is very good. Correct CCA, zero corrosion, full charged, tight, etc.
Now inspect the battery cables. ANY 'corrosion', replace, How old? >10 years, replace, Insure properly connected (clean and tight )
test the starter. using normal safety precautions, use a 2nd battery ,jumper cables and connect TO the starter, to confirm it spins. If it does, starter's fine.
NOW, get the wiring diagram and locate all the 'safety' switches, test each one for proper operation.Also inspect every inch of wiring for nicks, scrapes, chew marks. Check all fuses. Remove,test, reinstall.
Check that all levers and pedals with 'safety' switches operate those switches properly.