I have a 1996 Ranger that the Heater/AC blower only works when the outside air temp is above 45 degrees. I have replaced the relay, and the blower motor resistor. Any ideas what could be causing this?

Thanks

 

When i had this problem on my 87 it was a flaky connection at the connector under the hood. Removed it, from the blower motor plugged it back and never had another problem.

Try that and try wiggling the wires under the hood and dash.
And try giving the blower motor a good dope slap with your Ford wrench

 

Seems like in a older post here a fellow found a bad connection behind the passenger side head light of all places,,,but not sure on what year that was on,,,might be worth a look

 

I'd check on worn brushes and oilite bushings/bearings that are gummy. If the brushes go, there'll be less amps to create magnetic force contra the permanent magnets field. Increased temperature may allow the brushes to extend against the armature a little better, and conduct a bit better. Of course it could be the switch, the resistor pack, or the wires & connectors involved. Sometimes just disconnecting and re-seating connectors will make them work better.
tom