VW brake lights

VW owner: 78 Westy. My brake lights only come on when the headlights are on. Any clues?

Intelligent Tinkering: There will be somewhere be a crossed wire,  failed connector, or, if you or someone has messed with it recently, an incorrectly wired connector. If someone has messed with it, most likely under the dash in or close to the fuse box, otherwise most likely in the rear light units, or possibly on the master cylinder. Need a 12 V test light, a VOM with continuity setting, a logical mind, and some patience.

(Later, after the owner changed his question to state that the brake lights just won't come on at all.)

This is just a brake light failure. Tails and heads are nothing to do with it. We assume you've checked the fuses. If so, use a bit of two by four to wedge the brake lights on against the seat pedestal, then look for 12V with a test light or VOM at the bulb. If you find it, suspect ground or bulb. If you don't, suspect the two switches on the master cylinder. One hot in each is for the brake warning light on the dash, the other is for the actual brake lights. Pull off the two three prong connectors and search for 12V with the test light connected to a known good ground. You can find the pin-out online. If they don't have 12 V then you have a bad wire back to the fuse. If you find 12 V, jump the switch to see if the brake lights come on. You only need to jump one, since they are redundant. If the lights come on, replace the switch (s). have the new one (s) ready before you take the old one (s) off or you'll be bleeding brakes.