Where do you start if the rear defroster on a 1999 Yukon only defrosts one rear window?

If the vehicle is equipped with barndoors, there are copper contacts on the door jamb and the door itself has several pins all in a row for the defroster. I had the same problem with my 97 Suburban. Clean the contacts with electrical contact spray and a q-tip, and make sure the pins aren't frozen . If very corroded use a small wire brush. If one side is working you can rule out the switch or relay.