grab a multimeter, set it to ohms(resistance), disconnect the vehicles battery and set it aside. check the starter wires from end to end (as in, positive at one end, negative on the other from your multimeter) a wire can fill completely with corrosion and look 100% fine on the outside, and allow almost no amperage to run through it, which WILL cause your problem. do the same for the ground wires and such. its fully possible that corrosion will allow your multimeter to show 12V going through it, but so much resistance from corrosion that youll have nowhere near enough amperage going to the starter to start your car. no multimeter? grab a set of booster cables. go from the positive battery to the positive post on your starter. try it. no go? try the negative on your starter to a good known ground, or negative post. try it again. this is a very good place to start, the corrosion gremlins can be very sneaky and evil. good luck my friend. source:12 yrs auto technician