It is impossible. Buy a new one. Don't waste your time!
I found a good fix @ http://www.findonefindall.com/Toyota-sienna/ Please contact Toyota (phone number in your owner manual) to document this defect, the more complaints they receive, the better the chance of a recall.
A more complete URL to the page referenced in the second answer is ...
I used that successfully to repair the broken rear door latch on my 2000 Toyota Sienna. ("Buy a new one" is ~$65. I made the repair with tools and screws lying around the house in about an hour's time. Repair at the dealer reportedly is ~$300.)
Where is the short at? What is malfunctioning?
I would check with Toyota and make sure its not covered by recall. Our 2004 had both of them fail and Toyota replaced them for free.
Upon further inspection, the rear door handle assembly on the 2002 Toyota Sienna IS the same as that shown on the Find One Find All website, so never mind.
The following link has instructions, diagrams and pictures showing how to make this repair easily and inexpensively: http://www.findonefindall.com/toyota-sienna/siennareardoor.htm
check out this website it tells you how to repair the handle since it cost about $65.00 to buy a new one. http://www.findonefindall.com/toyota-sienna/
You can get a replacement part from RockAuto for about $65 and have it locally installed for under $100.
As far as I know, Haynes and Chiltons haven't published their manuals covering the 2004 Sienna. So the only one available appears to be Toyota's manual, available from any dealer's parts department or on ebay. From: Mark in Ohio (posted 5/12/08)
Next to impossible. A new release lever will need to be installed. Take it to the Toyota dealer ... they built it, they know how to repair it.
Remove inner door panel for access
There are pages of things that will set a code and turn on airbag lights. Take to a repair shop and have diagnosed. Airbag systems are not for the diy'r
Toyota has issued a technical service bulletin for seat heater replacement. It seems that this is not an uncommon problem. The seat heater has been redesigned. You can find the bulletin here: http://ww2.justanswer.com/uploads/jarrid191/2008-10-20_123630_T-EL002-06.pdf or search on "toyota sienna seat heater problem"
A repair manual for a Toyota manufactured vehicle can be found through the Toyota website. Free service manuals can be found online at the website 'Repair Service Manuals' under Toyota.