This is the brown switch, about the size of a deck of playing cards (approx) on the firewall? If so, there should be a cable that runs to the transmission. This is the neutral lock out switch. It allows the connection to be made for the starter with the car in either park or neutral. If the car is running, the switch is out of the circuit, consider when driving, if the transmission is placed into drive, the car continues to run, as you have seen once you start the car and then move the lever.
This is how it works for my car, which is an automatic transmission. I don't know how the manual tranny set up works.
I am traveling and am not able to see which position is which (lever to the top, etc) but I am sure someone else will be able to answer.