Thanks everyone for the helpful ideas.
Yes, I was already at the police station earlier today, to report it stolen.
But I'm looking to keep the car from ever leaving my property without my permission again. The other party is so mechanically deficient that any of the above would work. Wish me luck!

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1991 300E 124.030 103.983KE 722358 03 412178
207K

1979 240D 123.123 250K (Project car)

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2015 Dodge Ram 1500 EcoDiesel 37K