Martin,
It goes up at an angle from below left just where the ash tray (remove ash tray and mount 4 screws) and remove radio if you have one. Also leave loose but don't remove the glovebox light receptacle (tighten it later, make sure the bulb is good). Collapse the cardboard box a little (squish it a little without creasing or damaging it) go at it from that angle, then rotate it into place.
It's a PIA for sure, and a knuckle scraper because of all the sharp edges.
After you're done, quit for the day and come back to it another day when you're in a better frame of mind.
Oh, another thing, before you install the heater assembly, test fit the cardboard glovebox (easily done without the heater in place) as there are different qualities of cardboard inserts. I bought one that didn't fit worth a damn. Buy a good one!! And make sure it fits.
Cheers,
~Earl J