Wikipedia gives a good explanation of why plastics get brittle as they age: 

Plasticizers for plastics are additives, most commonly phthalate ester. Almost 90% of the market for plasticizer is for PVC, giving this material improved flexibility and durability. Plasticizers work by embedding themselves between the chains of polymers, spacing them apart (increasing the "free volume"), and thus significantly lowering the glass transition
 temperature for the plastic and making it softer. For plastics such as 
PVC, the more plasticizer added, the lower its cold flex temperature 
will be. This means that it will be more flexible and its durability 
will increase as a result of it. Some plasticizers evaporate and tend to
 concentrate in an enclosed space; the "new car smell" is caused mostly by plasticizers evaporating from the car interior.

Rich
San Diego

--- On Thu, 1/19/12, JordanVw@AOL.COM <JordanVw@AOL.COM> wrote:

From: JordanVw@AOL.COM <JordanVw@AOL.COM>
Subject: 25 yr old plastic bits in the winter.. wasRe: 1986 Westfalia Rear              Defrost Switch
To: vanagon@GERRY.VANAGON.COM
Date: Thursday, January 19, 2012, 7:59 AM

In a message dated 1/19/2012 9:11:02 A.M. Eastern Standard Time,
mehammons@GMAIL.COM writes:

Does� anyone know if you can replace the light in the rear defrost
switch?���If so how do you get to it without destroying the� switch?

yes u can replace it..� but be careful taking your dash cluster out� this
time of year ...those things are fragile enough as it is, and with the� cold
temps 25 yr old plastic likes to shatter in a million pieces...i would run
the van for a while and let the interior temps get to at least 75 degrees
(including interior surfaces)

the warmer the plastic is, the more flexible it is.���the colder� it is,
the more likely it will crack.

ive even used a hairdryer to heat plastic in the immediate area i am
working on..