Here is what I would do...might be a better way but it's the way I think.

1) have your daughter apply the brakes with the key on while you are in
the back.  Observe if both brake lights work or don't work.
2) Pull the tail light bezel out on the one that doesn't work or if both
don't work then either one and check the bulb's filament
3) check that the correct bulbs are being used and that there are actually
bulbs in the places that the brake lamps should go...this might be obvious
but we all hear about crazy things having been done or overlooked.
4) if filament looks good, check for voltage at the bulb socket while
applying the brakes.
5) if 12V there, check and see if the voltage drop is good, i.e., a good
ground there.

Beyond that, I would need to check the schematic and start tracing back.

Pat

From:   Jim Felder <jim.felder@GMAIL.COM>
To:     vanagon@GERRY.VANAGON.COM
Date:   10/08/2015 12:27 PM
Subject:        Help me diagnose and fix no brake lights
Sent by:        Vanagon Mailing List <vanagon@gerry.vanagon.com>

Daughter was driving behind me in the Westy today and called me to say
neither brake light was coming on. All other lights working. Brakes
excellent, newish MC and brake light switches.

All fuses look good. Where should I start looking?

Thanks in advance for your help. I need to head out of town overnight next
friday.

Best,

Jim