RepairPal Certified shops can diagnose and repair your 1999 Ford Explorer. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.
if its not the thermostat then its always worth checking out the heating core to see if its clogged. I usually blow on one end of it and see if i can get the coolant to shoot out, if it doesn't. it might be clogged.