In 2003 there were no heated seats if you wanted cloth. I can't recall the option code and I'll check the service manual later today if noone has come up with the code. If it a Custom model? or Limited? Do you know which option package it had?
Do the buttons light up when you press them while the motor is running?
You can spend some of that quality upside down time looking under the seats. The seat heaters involve a small box control--look for wires going up into the seat.
The easier way to do it would be stop by your friendly Buick dealer and ask them to run the VIN and tell you if it has heated seats and why the door panel has the buttons...