Best place to run a wire through the firewall on the driver side

I am running a new system in the car (Mercedes 1979 300sd) for the glow plug relay and I need to run a wire through the firewall to connect a switch and another one from the switch back to the fuse box. I would really like some input into where the best place is to do this. Thanks in advance. I would like to mount the switch somewhere on the left hand side of the dash if possible.