It is glued on with weather strip adhesive. When I pulled one off of a dart I had you could see where the just kind of s shaped the way the squirted it on and then stuck it up.
And it's a PIA to get back in there and around everything. :D
I just put it back all the way up to the top of the firewall and didn't even bother trying to get it all up around the wiper arms and stuff.
Just enough to block engine heat.