There have been threads like this in the past with all sorts of neat ideas and tricks that I think are ultimately of little value.

If someone really wants to steel your vehicle they can and will.

To keep the casual person from jumping in and taking it for a spin lock the doors and take the key out of the ignition. Any more than that if someone is determined to steel it they can figure out a way.

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-