ignition switch?

The smallest wire on the starter relay (yellow) is the start signal wire.comming from your switch.Check it with a test light. Should light up when you attempt start. If it dont the switch is the probem. If it does light up then the neutral safety switch on the transmission isn't providing the ground needed for the relay to work. That would be the larger slip on wire (black). Use a jumper wire from that relay terminal to any ground and the relay should work. Have you tryed starting in neutral position ?
hope this makes sense.