You say you are testing voltage. You may be getting voltage but NOT amps. To make that motor work you need AMPS and volts.
For example: The glowplug relay in my truck was passing voltage but the truck would not start. It could not pass enough current to heat up the glow plugs. Replaced the relay and whamo it worked perfectly from that point on.
My guess is that you have a switch that is not passing current or a wire that is partially corroded/broken. Look down that road and find your culprit