I have a 67 cat with factory air. I could not find any forum answers on how to install a new defroster plenum or the center radio speaker. So after many struggles and some wonderful help I got them in. Here is the sequence to install them.
First remove
glovebox, center console, radio, dash panel, instrument cluster, seats, heater defuser on the bottom of the blower box. If the heater core or AC evaporator need replacing, do it now.
Either remove, or loosen to the maximum possible all 3 bolts for the blower box, 2 in firewall, 1 inside.
If the original speaker and/or defroster plenum are present, remove them as needed.
Put back in
center speaker. Tighten them down evenly on both sides since you are squeezing a gasket to seal opening.
Finagle the defroster plenum in from the instrument cluster opening, while your other hand (or a helper) puts a hand up the glovebox hole to guide the plenum into place. BE GENTLE!! The studs that hold the speaker and plenum are small 10-24 screws that are attached to small clips that snap into the top of the dash steel. They are easily dislodged. If one comes loose you can put it back. Get a closeup camera or borescope so you can see what you are doing there. You can’t see enough while lying on the floor with you head stuck up an impossible place.
Make sure the speaker wire is on the left side.
Install the plenum. 4 screws on clips. Use the flanged 10-24 nuts. Torque is 3 fingers on the head of a ratchet handle with a 3/8" long socket.
Retighten the blower box so it snugs up against the bottom of the defroster plenum, and compresses the foam gasket there.