suggest a varistor

I've designed several circuits that interface with door lock actuators.  You can never have too much protection on an input, but it sounds like you are over complicating things a bit. 

All of my applications have been interfacing door lock signals with 5vdc microcontrollers.  A typical circuit consists of a diode on the input to prevent the input from going too negative and a large bias resistor driving a typical NPN transistor.  The output of the transisor then interfaces with our 5vdc logic and works pefectly in our commerical application that has been around for 2+ years with a very small failure rate (and no failures on the door lock circuit).

I've never used a CMOS chip in a car.  Like I said, everything we do is 5VDC, so we use a regulated, filtered power supply to provide a consistent voltage (the single most important part of automotive electrical design).