Attach the wiring to everything in the dash fitting it in the rubber coated hooks in the dash frame. Then when you set the dash in place you plug the bulkhead in at the firewall, and connect minor things like the power feed wire for the heater box, body harness at LH kick panel, brakelight wires, defroster ducts etc. The factory built up the dash frames with all the gages and wired them up before installation. It's the easiest way to assemble this into the car. Think like an assembly line.
What's easier loading the dash into the car assembled and wired, then plug in a few connections afterwards like the heater power wire, and firewall bulkhead connector and stuff like body harness at the kick panel etc? Or loading the dash into the car and then laying on your back on the floor and feeding the harness into the car?
Set the wired up dash assembly with gages installed on the lower 2 mount bolts, and while its tilted back with somebody holding it for you, connect the power feed wire to the heater box wiring, and bulkhead connector to the hole in the fire wall, connect heater box cables, and defroster ducting to the vents, then rotate the dash in place and install the upper mount screws, and tighten the lower screws.
