I believe behind and close to where the heater box is secured to the firewall then down to the passenger kick panel area. There should be a mounting hole for the screw.
Before we took off to our cabin I gave it a look. You are right. The cable should be routed close to firewall between the hearter core and the right air vent. It just fell through and I found the hole for the screw as well. Thanks Don!:thumbsup: