Spray it with some lubricant and use a very small screwdriver to pry on the rubber seal to cut it or move it out. I did it this way on my 420SEL and it worked. After you break the seal you will be able to remove the thermostat with your hand. Before that I tried turning it with pliers but to no avail.

Roman
87 300 SDL 158K
86 300 SDL 216K
86 420 SEL 227K
81 300 D 148K