There isn't a single, dedicated "door lock relay" in the traditional sense on a 1998 Chevrolet Lumina. The door lock operation is controlled by the Body Control Module (BCM). The BCM receives signals from the key fob, door switches, and other sources, and then sends signals to the individual door lock actuators. There may be relays *within* the BCM or associated with other power circuits that the BCM uses, but there's no separate, easily identifiable relay specifically for the door locks.
To troubleshoot door lock problems, you'd need to diagnose the issue using a scan tool to check for BCM fault codes or by testing the power and ground circuits to the actuators themselves, and checking the switches. A mechanic or experienced auto electrician will have the proper tools and knowledge to diagnose this effectively.
