where is the break light switch on a 1984 vanagon

The Vanagon uses what is called a "pressure switch". Two of them, actually. The switches are activated by the fluid pressure inside the braking system. The switches are designed to activate at a given pressure. With age, it takes more and more pressure to activate the switches. Installing new switches will allow the brake lights to come on at lower pressure than the worn out old switches, meaning that it will take less pedal pressure to activate the switches and thus, the brake lights. Most cars have a simple button under the petal, but VW got carried away on this I think.

It screws in to the master cylinder