nope not really concerned..

this is a working van, like a working dog it's in any & all weather.. 
and well weather tends to migrate into the van in many places.. 

we drove thru some heavy rain on our maiden voyage, and while I can't say it'll never leak.
it didn't leak.. while the overlap is facing forwards, there may not be much side pressure from the wind in that location and there is a lip to the upright that'll divert rain as well.

if I had the time I would have ordered new whisker seals, but this was a race over 3 nights to get installed for the 2hour drive we were taking friday. 

Dan

________________________________________
From: Mike S [mikes@flatsurface.com]
Sent: Monday, August 01, 2011 12:50 PM
To: Daniel O Stevens
Cc: vanagon@GERRY.VANAGON.COM
Subject: Re: Rear Sliding window modificaiton - Success

At 12:02 PM 8/1/2011, Daniel O Stevens wrote...

>I made this to be installed backwards to allow the rearmost section to
>slide,

So, you're not concerned about water infiltration when driving in the
rain due to the way the window panels overlap? Water would tend to get
forced between them.