Anti Thief devices.... what works

All you're going to do is keep some kid from stealing it to joyride it. Lock the doors, keep valuables inside the car out of site, install a battery disconnect switch and/or an ignition kill switch, fuel pump kill if you've got an electric, etc. And don't leave it parked in a trailer!!!

If those things won't stop your thief, they're likely a professional, and you're not stopping a professional. Most classics are not stolen "gone in 60" style. They're loaded onto flat beds with wheel dollies. No hidden switch, disconnect, wheel lock, brake lock, open exhaust cut out or anything else is going to stop that.

There's a story on the HAMB about a guy having his classic stolen right out of his driveway. Other cars parked in the way, up a steep driveway off the street, with him sleeping on the other side of the window it was parked next to. Wheel dollies and a flat bed, buh-bye. If those guys want it, they're getting it.

And nowadays your casual thief isn't going to steal a classic anyway. They're too loud, they draw too much attention, they're hard to drive, and you never know what custom deterrent or mechanical problem you'll run into. And then, once you've stolen it, the parts are too hard to move without drawing attention. So really you're just trying to stop the local kids and drug addicts, and that's not that hard.