2001 ford fiesta car-van. 170000 miles approx. Over the last couple months the heating wont work unless I do about 70mph. Changed thermostat and heater control valve. Had look behind dash in car and all seemed fine except two bulbs were needed. All the pipes were blown out by mechanic. Voltage seemed fine at heater control valve but. The pipe going into the van had only hot water and was cold coming back out! Any ideas before I freeze in this van!