[Hutch]

BCOWANWHEELS
Check the ground wire from the relay inside the cab on the firewall.
It is very short
It is located up high between the glove box and passenger side wall.

There is a short ground wire that is supposed to go one of the shroud bolts that come through the firewall
Easy test is to attach a short piece of wire while it is on high from the relay to ground.

A lot of folks do not get the ground re-attached correctly while working on the heater coil.

The symptoms are
Low= Low
Medium= Medium
High = Very SLow