I'm working on a keypad that's from an old cordless phone.
The keys sit on a printed pcb with all the proper connections... I think it'll be much more durable than a normal keypad and there's no membrane
(the really cool thing is that it uses almost exactly the same common connections so I only need about 8 wires to connect to the Keyboard controller.)
Also, you might want to find a way to make it easily removable. the heat in a car could easily breakdown rubber...