If you are getting 12 volts to both lugs then your glow plugs are likely burned out. You can pull the harness and ohm them out.
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2003 F-250 XLT Crew Cab 7.3L, Chrome BigTex Grille Guard, Quad pillar - 3 ISSPRO gauges (trans, pyro, boost) and DP-Tuner F6; Roush fuel pressure / temperature / oil pressure gauges, Ford Severe Duty AIS, 31 row 6.0 transmission cooler, ScanGauge II, Marinco mod, Walker BTM
"IF YOU CAN'T FIX IT WITH A HAMMER, YOU’VE GOT AN ELECTRICAL PROBLEM"