Second Gen Sienna have a known issue with rack and pinion seals... they leak on the passenger side and the fluid gets collected by the boot. I think it most often fails on the bottom or at least lower half of the boot. Lots of posts about it on various sites. I have to deal with mine in the near future. Some people have had success adding a specific type of power steering fluid (I don't recall the type) that keeps it from leaking at the seals. Others have purchased refurbished rack and pinion parts. Again, lots of posts about it.. But once the boot is damaged, it has to be replaced. Please let us know how you proceed and the outcome.