About the Sienna horn working randomly (sometimes while moving,sometimes parked,sometimes not at all etc).
As several people mention before,is due to a faulty ground. It seems that the ground for the horn comes through the steering column.
Dirt built up and dried out grease in the joints,or rust; or a combination of all creates a a scenario where the ground for the horn in faulty.
WHAT SOLVED MY PROBLEM !!!
after reading most of the postings here I concluded that cleaning the steering shaft joints should solve the problem.
There is a joint right below the dashboard.(move the driver seat back as far as you can get under there and with a rag and a can of WD-40 clean that joint by spraying and wiping as good as possible.(be generous with the WD-40).
Also there is a bearing assembly at the floor(firewall). Spray enough WD-40 there too.
The steering column has three positions for the steering wheel,adjustable by the lever on the left side of the steering wheel.
I drove the Sienna right after using the the WD-40. changing the steering wheel position several times and using the horn many times to restore the ground contact that I just cleaned with WD-40(Remember the electricity flows through the joints we just cleaned).
At the first few tries the horn started working more often than not and after a few minutes of changing the steering wheel position and honking the horn worked all the time !!!.
All You need is WD-40 and a RAG.(Maybe some other contact cleaners,degreasers may work).I had WD-40 handy and it worked.!!!
Good luck to You all. June 2010