Help needed installing a Tail Light Assembly

[Aurelian]

Hi everyone. Nice informative forum you have here!

I just removed a broken tail light assembly on an 1983 Mercedes 300SD.
I have the new one ready to install, but can't seem to make it fit snug, in fact as i try to install the new one and screw down a bolt the other end will stick out about 1 1/2 inches.

What is the proper way of installing a tail light assembly in these cars, as i don't want to hammer it in and then break the new one?

Is there a trick for putting these in? Do i put it long side first? short side first? both at the same time? and how do i push in the seal without breaking the plastic.

Your help is MUCH Appreciated!!!
Thanks,

Adam