Hi Nick,
Just to be clear. On my 75 Carrera, BOTH wires connected at the starter solenoid are yellow.
On the blower motor relay pin 87 (output power out, normally open) is yellow/black to the 14 pin connector (male, bullet) at the back of the rear left corner electrical panel. From the corresponding female 14 pin connector pin 87 is black. That black wire is now the switched 12V supply to my EFI electrical panel.
14 pin connector for engine wiring harness:
EFI eletrical panel:
The big red wire is the switched 12V supply and is connected the black wire from pin 87 of the blower motor relay:
Good luck with your project!