In one of my cars I had someone on two different occasions break into my car to try to steal my stereo (my silent alarm went off and paged me). I ran them off both times (it must have been disturbing seeing a big fat man like myself in my underwear with my 10mm Glock with the laser on flying out the door). However, I saw a common theme, they always pulled down the covering on the bottom of the dash and was doing something on the back side. So, when I repaired it the last time, I riveted a 10" knife blade to the frame of the lower dash hoping they would reach around it and get cut. It never got tested again unfortunately.
BTW, I wouldn't think about doing the knife thing now with liability being what it is in this day and age.
I actually had one car with a bad electric seat motor that I wired to a relay (some what like above) but using just the single not working switch.
Another thing I had done is I had one of my radios wired hot all the time and I pulled the face plate when I left the car. I used the remote turn-on lead (normally used for amps) as an ignition enabler. Worked well...
In the next few weeks, I am putting back in my old Alpine alarm with all of its bells and whistles. After they are done putting it in, I am then going to go in and put in some type of hidden kill, reed, or similar switch. The only thing I haven't figured out yet is how to make some form of valet switch so I don't have to tell everyone my secret spots. Anyone got any ideas for a way for a temp bypass? I think the reed switch would be easy, you could then just leave the magnet in place. However, it might look odd if it were easy to get to.
I still use the club, I lock my glovebox, I lock my keys and existing alarm remote into my 1000# gun safe, and have it insured and am paranoid where I park when I do drive it.