Is the relay (and the socket) of 12 pin configuration?

If an O2 sensor is installed, the heater will be powered from from socket number 7, which corresponds to pin 87 on the relay. If you examine the bottom of the socket, one wire on 7 = no O2, two wires on 7 = heated O2.
In the event that there is an O2 sensor, disconnecting the power lead at the fuel pump will allow you to search for a ground fault in the heater wiring.