2003 Sienna remotes intermittent

Greetings!

The remotes (keyless entry remotes) just started to only work intermittently yesterday. We can typically open/lock the doors from 15-20 feet away, but when the problem crops up, the remote will only work from within 1-2 feet away from the console.

I put new batteries in the remotes and there's no change. I think it's still under waranty, but I figure I could save myself the hassle of bringing down to the dealer, etc. if I could track the problem down myself.

Can anyone tell me where the ECU is located and/or whether the antenna for the unit is self contained or external?

Thanks for any help or advice!