RepairPal Certified shops can diagnose and repair your 2004 Mercury Mountaineer. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.
If you have hot hoses in and out of the heater core, then you have a temperature blend door motor issue, or a cracked plenum door. Either way the dash needs accessed and pulled back.
Can you give me an idea as to where to look behind the dash. I have the same issue on 2005 mercury mountaineer.