Just the heater circuits? Find the two matching colors on the o2 sensor side of the harness. Most of the time, 2 white wires. Check for power with key on, cold start. If there’s power, disconnect and ohm across the 2 wires for an open. If it’s open circuit, o2 is bad. If there’s no power to heater wires, note the harness side colors, get a pinout of the computer and test at the PCM.