It sucks regardless. Back in the day, we would mark a square area, cut 3 of 4 sides with a whiz-wheel, peel it down, replace the dead fan blower motor or heater hose, push it back up, seal the gap, enjoy the heat again. I always tried doing it the factory way first, but rusted and corroded bolts with rounded heads inevitably made the owner make the decision. Absolutely horrible design.
PB Blast everything for at least 1 day before. Start a timer, document it so we can see it, change the blower motor while you are there—trust me you don’t want to do it twice.