[dandaman]

Can you hear the clicking while it's installed and someone is toggling the switch? The defogger is a pretty simple deal, you have two points of failure:

1. Input into the relay (Body Computer powers up the coil)
2. Output of the relay (Fused power to the defogger).

#1 is easy to rule out by hearing it click installed. #2 is a little more tedious and takes a volt meter and probing the various connectors.

Looking at the wiring diagram ( http://minivanclub.org.ua/info/Sienna/Sienna2005RepairManual(RM1163U)/44/systemci/062fwdrw.pdf ) there are a couple of junctions to test out.

I'd also do a visual check of the connectors to the wire array on the back glass and make sure they're still connected. That's been the usual point of failure for when I've seen defoggers go out. That said, I haven't looked too closely at my (new) Sienna but I'm not sure exactly where those are at.

 

[dandaman]

Not sure but the relay should be marked, it will be the 85 and 86 terminals. If not this diagram should help you identify them:

http://www.the12volt.com/relays/relays.asp

 

[dandaman]

You can try to swap the relay with a known working one. You can also bench test it by taking a 9V battery and applying it across the coil terminals, if it works it should click.