98 Sienna horn works sporatically, dealer has no idea, replaced the clockspring, still works intermittenly. Seems to work better when car is still. Hitting the steering wheel in the middele sometimes helps.
Replace dealer. Have independent shop check horn switch, itself.
This probably wont apply to your Sienna, but on some Fords the wiring to the tilt wheel can pull & lose connection when the wheel is full down.
Try this simple little test, hold the horn button down & move the tilt wheel up & down. Does the horn sound off?
As hellokit mentioned, ditch the dealer & find a GOOD independant shop.