Doug,
The white wire connects to the bullet terminal of the ignition key housing and to one terminal of the door switch.
The other terminal of the door switch has a green wire that connects to the key buzzer.
The other terminal of the key buzzer has a gray wire that is spliced to a red wire that comes from fuse 7. That red wire also feeds the horn relay and the A/C condenser fan relay.
FWIW: Bill Taylor's diagram shows an orange wire spliced to the red wire, and I have a photo of a '74 relay panel showing a brown wire instead of gray or orange.
The white wire at the horn/turn/dim switch connects to the red wire at the headlight switch. Oh no....not another RED wire !!! 
There you have it, more De Tomaso confusion. 
John