1992 Buick Park Avenue Evaporator Removal

[robmaxfli]

Hello,

I am at the final stage of removing the Evaporator but there is one more bolt that is preventing the whole case from being detached from the firewall.
It is right in the middle of the case at the very bottom. It has a white plastic retainer on the threads. I have gone into the interior of the car and have already removed 2 of the bolts thinking that surely one of them was the correct one -

I went to the library and none of the books show or discuss this...I cannot seem to find the bolt that is holding this evaporator case to the firewall....I am at a loss here. Any pictures would be wonderful.

thanks!