Michael,
 
Voltage regulator but also check the wiring harness espcially where it passes into the engine hatch.  Look for bare or warn insulation and loose connections if you have a trailer hitch set up as well.  Also if the boots on the light sockets are warn water can leak in and cause a short.  I've seen problems here on Zeno in the past that resulted in lites burning out before their time as well.
 
2 cents,

-- 
Matt Thyer

 
> Date: Sun, 24 Apr 2011 21:51:35 -0400
> From: mikes@FLATSURFACE.COM
> Subject: Re: Tail Lights & Good Samaritans
> To: vanagon@GERRY.VANAGON.COM
> 
> At 09:28 PM 4/24/2011, Michael Rasmussen wrote...
> 
> >On the chance it's not the bulbs, it's the _____, what should I check
> >out?
> 
> Voltage regulator. Check the voltage while revving the engine.