1995 GMC Sierra

There isn't one. The purple wire at the starter solenoid should be hot with the key in the crank position. If it isn't, backtrack the circuit. Make sure the battery has a full charge and connections are clean and tight.

The autostore sells a remote start button it hooks to the starter solenoid. Maybe the leads are long enough so you can test without being under the vehicle? There are safety issues trying to jump the starter, make sure the tranny is in park or neutral and a functional e-brake is set. If you crank it and the vehicle is in gear, it's going to move, run over anything or anybody in the way. Safety is your responsibility.