Father-Son 1974 Duster Project

I got the heat/ac box in the car today. I put a thin coating of lithium grease on the seal to the cowl vent just for grins. There is a small hole in the firewall near the heater core penetration, and I'm wondering what goes in or through it?

I also cleaned up our under dash harness. Then I layed down on the seat and looked at how to put it back in. Its been so long since we took took it out I don't remember how it routes around the steering column or where the fuse box goes or what holds it to the fire wall....