glow plug relay not working on 97 ford f-250 diesel

thats the problem, not the relay. you need to find out why the power side is not working. it could be a fusible link or broken wire. then you need to find out the reason for the failure or it will happen again. Roy

Besides taking it to a shop, you'll likely need the service book to test the wiring. If the relay isn't be activated when in KOEO position, then it's likely the ignition switch is bad. Always ensure perfect ground and power connections, typical domestic vehicle issues.