It's often a small, rectangular or square-shaped component, sometimes attached to the blower motor housing itself or nearby on the ductwork. You'll likely see wires connected to it.
Before you start: Consult a repair manual specific to your 1993 Chevy G20 van. This will give you the most accurate location and removal instructions, including pictures. A Haynes or Chilton manual would be a good place to start. Searching online for "1993 Chevy G20 heater blower resistor location" with images might also help you visually confirm its position.