Another option is the negative battery terminal quick disconnects. They have an inline 3A fuse so your radio and clock won't lose power. Now someone might see you fiddling with it in the trunk but if the disconnect is in your pocket it won't do them any good.

I agree with the comments about the flatbed. Personally, I've always subscribed to the theory that you will not stop someone that really intends to steal YOUR specific car, so the best you can do is to treat security like the problem of escaping from a bear. You don't have to be the fastest person, you just don't want to be the slowest. Most thieves are opportunistic by nature so unless they have specifically marked your car, they are going to go for the easy opportunity.

Heck, I've wondered if a car thief under the age of 20 would even know where to start trying to break into and hotwire an old 911!

Wire a 1/4" phono plug into your fuel pump circuit. Mount the female portion in your dash and keep the male end with your keys. Very unobtrusive and who the heck would expect it?

hide the switch under the dash somewhere or in front of the console. the easiest wiring route for a CIS car is through the cowl past a wiring grommet or heat hose and split the fuel pump brown ground wire where it goes through the smuggler's box. carreras have the DME relay under the driver seat. find the ground wire to the relay and split the brown ground wire. hide the switch on a small bracket at the front of the seat rail, down past the carpet. use a quality marine metal toggle switch.