Well, theives aren't movie characters with every high tech gadget in their pockets. They're going to be poor and have routines for specific models. Just about anything is going to discourage them, but something like a slimjim cover is going to make them break the glass. Custom color coded wires hanging out under the dash are an invitation. They can also cut the battery cable from under the car to tow or strip there. I don't think what the car looks like matters so much. They either want to break in and rip (destructively) valuable stuff out quickly, or steal the car to chop. The bad thing is, if they can't take the car, they take everything else that they can.