battery replaced every year

Parasitic draw test is easy. Disconnect one battery lead with the ignition switched off keys out of ignition. Put an Ampmeter in series with the disconnected battery cable. Read the draw after 30 seconds or so it should be under .2 Amps, if not there is a parasitic draw. Also have the charging system checked. I have seen alternators unable to reach max output. If the alternator is rated at 70 Amps output and only puts out 50 or 60 amps the vehicle would constantly draw on the battery. The Sienna isn't a complex charging system. If your interested in reading some technical articles check out http://www.autoshop101.com/

Yeah, replacing the battery every year is crazy. That is completely the wrong thing to be doing. I recommend finding yourself a new shop--a quality independent that knows Toyotas in your area will be able to help you diagnose and permanently fix the problem. It could be something draining the battery--maybe something.