the purple wire on the starter is what pulls in the solinoid. that gets pwer from a relay pulled in by the key (switch). check that the relay is clicking by putting your hand on it while trying to start. If it is, check for 12v+ at the purple wire on the soliniod while turning the key. if so, solinoid is dead. if not, relay is dead. check to see the relay is getting 12 from the key when turned. check the relay ground.

a bad block ground can cause this. do you have a large cable that connects the block to the chassis? is the battery grounded to the block or the chassis?