You only have between 20 and 30 seconds to do everything you need to do before the nav locks up the functions, so if you are that fast (and the NAV does not hang like it tends to at times), then you only need a switch to kill the signal.

I plan to start installing the speed pulse generator this week some time. Won't probably happen tonight as I go to the gym on tuesdays, and tomorrow night I need to run some erands after work, then thursday I need to pick my wife up at the airport, and friday we are picking up our new puppy. Dang, I need to find some time to tear the dash apart.