If they really want your car, they'll probably take the keys from you at knifepoint in your living room. Thankfully unlikely with an old Zed though.
A lot of time nowadays, car theft will also involve the crim breaking into your house to get at your keys. So keeping your keys somewhere that's not immediately obvious is the first step. Next, I'd look into high security locks and door handles for your house. You can often fit these yourself with a bit of research, and they remove an easy, silent way to break into your home. Sure, they can still smash a window, but you're going to hear that.
Finally, as above, getting a large dog (or just any dog that's going to bark, really) is probably the best theft deterrent you can have.
