'77 models the switch is mounted on top of the steering column under the dash just in front of the firewall. On 78-79's it's mounted on the transmission. A very common problem with these was the pin that activates the switch would break or fall off.
The switch bolts to the column with two short screws, there is a pin that is clipped to the shift tube. The pin moves the switch back and forth as the gearshift is moved. This same switch also activates the back up lights.