If you can make it home with the car as it is, do so, and do the work there. Soak all of the thermostat bolts in penetrating oil and carefully remove the unbroken one so that you can get the thermostat housing off. It looks like the bolt broke off so short that there may be threads sticking out of the hole to where you can get a Vice-Grip® on them. Again, use lots of penetrating oil and work slowly. If that doesn't work, you'll have to drill a hole in the broken bolt and use an EZ-out.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970