Since I have not actually used the valve, I'm unclear on what is left in the car. Is this a threaded plastic rod that seats against something like a shower valve? If so, that suggests I could just spin the remaining portion out with an EZ Out and screw in a new one. On the other hand, is this more like a ball valve that just turns 90 degrees to open and close and thus the remaining part will not just unscrew?
I replaced my plug without any problem with the prongs breaking. For others who run into the same problem, I figured out a neat trick. The small hole in the remaining part of the plug was just the right size to jam in a T-25 screwdriver. It fit snugly and securely and allowed me to twist out the broken piece like a charm.