I was thinking the same thing Brian. I want to say a good while back there was a post with a similar problem. It's probably been a year or 2 since then.
Tried to look in my fsm (year 2000). No luck but i did find there is a diagnostic mode you can put it in to check sensors, conditions etc. no idea if it will work for a 99 (this is from a 2000 FSM) but might be worth a whirl. Does the temp seem to read accurately? Both in and out?