Back when I had my convertible mustang, the ignition starter switch failed. I was too poor to buy another ignition switch, so I looked around for another high load switch to use. The convertible top didn't work anyway, so I wired the convertible top switch to be the starter switch. Turn the ignition key on, and hit the convertible top switch to start.
Fast forward 3 or 4 years, and one day my mustang was stolen. I called the police. After a couple hours, the police dropped by and said they found my car. It was at the bottom of the hill I lived on, off the road in some bushes. The ignition wires were all cut up.
I inferred that they'd pulled the ignition switch and tried to short the wires to turn the engine over. That didn't work, so they tried to start it by coasting downhill and popping the clutch. My car had a manual choke and was a hard starter if you didn't have just the right touch on the choke. At the bottom of the hill, they just gave up.
I never thought that wiring up the convertible top switch to the starter would save my ride, I was just being cheap!
My brother had another lovely theft deterrent. He had a Ford Maverick, which was a column shifter. He hated the column shifter, and decided to install a floor shift. After much work, he ran out of time and had to head back to school, so he patched what he had back together - reverse and first were shifted with the column, and second and third were on the floor! To shift gears required juggling both levers right, with horrible clashing sounds if you failed. Putting both shifters "in gear" would lock the transmission. He was the only person who was deft enough (or daft enough) to successfully drive it.
My current theft deterrent system is to have major and essential components not installed the car, like the radiator, carburetor, ignition system, seats, etc.
