1998 Dodge Ram Heater problems

My heater isn't working correctly; I only fill heat when the blower is on the lowest settings, when the truck is warm and sitting still. I checked the heater hoses and both are hot going into firewall. I have replace the themostat; collant level is full. I have replaced the blower motor. Is there anything I can do before I take the dash out?

Thank you
Roger Ault