Yeah, you just need a diode - such as this.

A diode is a one-way valve for electricity - it will let 9 volts out of the 9v battery but won't let the car 12 volts reverse charge it and blow it up.

Or you can go to Super Cheap Auto and buy a ready made one:

There are wiring diagrams all over the internet if you google memory saver.

If for whatever reason there is an extra load (say a door light) on the 9v battery with 1amp 400v diode what will happen?

If the diode blows does it loose it's one way capability?
Would a 400v 3amp diode be safer?
Or would it just mean the 9v battery would get warm and drain quickly because it is supplying max capable current of 0.5 amps or whatever it is rated at?