Theft-proofing a convertible with no steering lock

Install a hidden line lock in the brakes that activates all 4 wheels.
You can't drag the car onto a ramp truck or tow it, let alone drive it away.
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Of course you can, silly. It's done by repo clowns every day of the week.

FWD in park? Parking brake locked? NO PROBLEM

Hell if the repo clowns are in a hurry and don't want to confront the "former owner" they don't even put the GD thing on dolleys. They just hook one end and slide 'er off down the street a couple -three-four blocks. Ruin the tires? What, they care? NOT!!