Extend the seat all the way up and carefully grind the head off with an angle grinder or dremel.

Get the seat out of there and assess the rust damage. On my '85, this consisted of replacing the floor pan, which I hope you dont have to do.

A threaded nut insert thingie (technical term) can be purchased from the dealer for not much, so dont bother trying to save it or weld it back to life.

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car