I have a 98 silverado, the air and heat will only blow out of the defrost if I set it to any other mode the air all blows out the bottom mid of the dash you can hear it and feel it whenever you switch the control off of defrost.. Anyone know what the problem is? And if so how to fix it. I'm thinking a melted or broken pipe or door but I don't even know how to get back in the dash to them

Mode door actuator is probably bad , check out youtube video .

https://www.youtube.com/watch?v=HRYHZoIO89M

That's done on purpose to blow dried air at the windshield to help it defrost faster. When in defrost mode, the AC is running. Standard feature on most cars and trucks

The mode selector is bad. IF you look at the plenum (where the fan motor is ) you should see two levers one is hot cold the other is mode. Check them by moving the controls and see if they move.
If the mode lever moves then the door on the inside is broken or something could have fallen in the vents and is blocking it.

the actuator to the defrost door is inop. try running the climate system thru all the modes ac heat defrost vent off etc, maybe it will begin operating . look for any vacuum leaks

Sounds like the blend door motor isn't working.

The blend doors are controlled by small electric motors.
One of the motors could have failed, or the blend door is sticking.

This model has a weak connector on the blower resistor. Remove the hush panel on the passenger side and disconnect the resistor. Look for signs of melting on the connector plastic near the ground terminal. The resistor may also have failed. I have the wiring diagram somewhere if you need it.

Your going to have to replace the switch. The switch it self may have gone bad, since it works on a vacuum. It's common for the diaphragm in the switch assembly to go bad, if it's not the switch. Then it's going to be a bad actuator door and will need to be freed up. I have found it to be the switch it self get dirty and the vacuum needed to activate the actuator door is not there.

Good luck and keep me posted. When you turn the switch from defrost to heater, do you hear a air leak??? If you do then it's a good chance that it's the switch it self.

will the air flow switch from floor to the mid to defrost? if so then we have a problem with the tempature door actuator. easy to replace just in tight place. Keep me posted for more help or if this was any help thanks

most american cars force the defroster to blow cold air, this is not a problem, it does this by design.