Doesn't matter what you do to the radiator.  The engine should be at full operating temperature within 5 to 10 minutes unless it's really crazy cold.  The thermostat completely removes the radiator from the cooling circuit if it's working properly, so covering the radiator won't give you any extra useful information.
I would clamp off one of the radiator hoses if you want a quick and dirty experiment to see whether the thermostat is doing anything.
-Hans

----- Original Message ----- 
From: "Ken Lewis" <kdlewis_wating_time@ALLVANTAGE.COM>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Thursday, December 15, 2005 11:57 AM
Subject: Re: no heat

I thought this would eliminated any variables from the experiment. Since you said it was pretty cold at your location, any breeze might be sufficient to provide cooling to the radiator. I have only heard the fan on my truck come on a few times so I suspect it is pretty efficient heat exchanger .
Ken Lewis
http://neksiwel.20m.com/
  ----- Original Message ----- 
  From: Jake de Villiers 
  To: Ken Lewis 
  Cc: vanagon@gerry.vanagon.com 
  Sent: Wednesday, December 14, 2005 9:48 PM
  Subject: Re: no heat

  Why do you need to cover the radiator Ken? Just curious.

  On 12/14/05, Ken Lewis <kdlewis_wating_time@allvantage.com > wrote:
    Jeff,

    I would cover the radiator and let it idle. It should heat up then. If it 
    doesn't then it isn't the thermostat.

    Ken Lewis
    http://neksiwel.20m.com/