Yellow Button

Near as I can tell, not a damn thing happens. I've started chasing the wires but lose them in the harness and passing through the firewall.

use your multimeter and test continuity. It may branch off giving you a couple different 'beeps', but it will narrow it down. One of the gurus will read this soon and pipe up. Im still thinking they bump the starter. Have you tried it with the key 'on'? and out of gear if a manual...