starter solenoid 328

The starter-solenoid sometimes won't engage when turning the starter key. It just makes a buzzing sound. Think the key operated switch has worn out.
Startermotor has had a check-up: like new.
My idea: intercept the wire and fix a second starter switch underneath the dash.
Question: which wire-colour do I have to look for? Is it possible to repair the starter switch? A new one is horror expensive because it comes with a complete set (doors, bonnet).