Even with the heater box removed, it aint easy
I have my in and out many times fitting a new speaker. I had to trim some (about half) plastic from the ridges on the inside part (that are used to divert water down). Then you angle the top edge in first with the rear edge of the vent in the hole, then push bottom and front in at the same time kinda wiggling it. skinned my fingers a bunch of times
then when the panel is mostly in, you have to pull the rear edge around the door opening edge. The you can screw it in. Dont forget the dumm-dumm putty on the LAST time you put it in, since you have to clean all the OLD putty off to get a good seal.
I am ALMOST done with my repro vent pulls project. I have molded 3 and going for the 4th. then to rewire them and bend the ends. Then I will be done with these $%&*@# kickpanels!
The kick panels have been keeping my car assembly from moving forward!!! They say 1st gens are built around the kick panels and I believe them