A switch is not just a switch.
Switches come in two varieties, Normally Open and Normally Closed, so you need to ascertain what type you have in your car before you run out and buy one of the wrong type. I would assume that the brake light switch that is controlled by the brake fluid pressure would be a N.O. switch and it closes when the brake pressure activates it.
Reed switches activated by a magnet come in both varieties so be careful about which one you use.
I would be more inclined to use a micro-switch with the arm on it. These have 3 contacts giving the benefit of having both N.O. and N.C. contacts so should suit most users needs. Available from RS for under £5.