I own a 95 chevy 2500. When you turn the blower on for defrost it blows warm air out,if it's hot outside. In the early mornings when it is cold it blows cold air out. So basically what ever the outside temperature is it matches it. Any possible suggestions? Thermostat? Water pump? Thanks David

Does your a/c work?

The defrost uses the a/c to dehumidify the air to clear the windshield.

The defrost control also moves the internal dashboard dampers to blow air to the bottom of the windshield.

so... there're two areas of concern.

Sorry forgot to mention Truck does not have AC.

There is a damper door that controls the temperature of the air. It mixes the outside air with air from the box with the heater coil.

Again there're two issues. the damper control isn't working or the heater coil is not circulating hot water from the engine.
I would check what ever is easiest to get to then go to the next ...