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An aftermarket power seat and the installation would be more expensive than getting the option with the car. HOWEVER A power seat uses the same mounting position as the standard seat. If you can pick up a power seat at the scrappers you could install it yourself. You would have to provide power to it though. I would suggest a 25 or 30 amp capable circuit for the seat. TRUTH - there isn't anything that the factory installs that you can't do your self if you have the cash.

Minister of Truth
Toyota's are bad enuf. To have an INDIE mess w/ their electronics BIG TIME? No way. Sure, the indie will tell u he can do it to get ur $. Then there's the warranty question. Pay for the original power seats, u'l be saving ton$. And only have the dealer service it, same reason.

UCANTCME
It can be done cheaper through an independent shop plus all of the needed wiring will be in place in the vehicle, so no additional wiring will be needed, just the seat with the controls..