IMO keep them tethered to you until they're 100% for an extended period of time. Say two weeks or so, to be safe. (I know it's a pain and sounds like a long time, but it's easier than starting over if they regress). Once they've met that goal, I'd go to off tether in a room only, with you watching them. Once again, once 100% reliable at that for a good period of time, add another small area to that, and so on. Chihuahuas can be hard to housebreak, but not all are. We had a chi growing up that was perfect, never had a problem. My Dad had one after I was grown and gone that was never 100% housebroken. My Dad's always had dogs, and never had a hard time housebreaking them, but Tammy Faye really was stubborn.

Pictures????