I had to rectify this problem in my 91 Silverado. Here how to do it;
First of all I am assuming you get heat to the floor, if this is true that means your blend door solenoid if functioning properly and that eliminates one problem (this chooses the heater core or cooling AC core). Secondly, there is a pair of solenoids stacked one on top of the other that actuate several doors in your heat/cooling housing. My truck pumped heat yet did not push it to the dash, first you need to locate the solenoids. If you lay on your back on the drivers side floor and look up under the dash with a flash light you may be able so see them (AS SEEN IN PIC) Sometimes there is a silver box 10" x 5" x 1.5" (AS SEEN IN PIC) that you may have to remove to see the second one. The 4 maybe errors are going to be (i) you have cut the power supply to the solenoid for deck power (ii) you blew a fuse (iii)when installing the deck accidentally the arm that actuated the door was knocked off the shaft (iv) the solenoid is fried. I have added pics I just took from my truck to show you the lower solenoid, the other is directly above it 6-8 inches, the pics will soon be on my profile page andI've added a pic of what the solenoids look like front and back.
To trouble shoot the error, you need to actuate the settings from floor to vent to defrost while looking from underneath, if you will see an arm with a spring on it floating around on the upper of the 2 solenoids then it has fallen off. This arm operates the door that opens a pathway for heat for defrost. I fixed mine by using gasket maker, I put the gasket maker on the shaft and slid the arm pack in place. If the solenoid does not move any arm it is an electrical problem, make sure the solenoids are plugged in, make sure the fuse marked "AC/Heater" is good, you may need to remove the solenoid to see if operate. NOTE; there are more than 2 hex bolts that hold the solenoids on.
Reason for this occurring after the aftermarket installation is your stereo was originally to the right of your instrument cluster. The installer would have had to access the rear of the stereo to either plug in the proper adapter to mesh with your deck (YAY) or cut the wires and splice in your new deck (BOOO). Either way they would have been pulling and working in and around these solenoids, hopefully they didn't mouse around and take a 12 volt source off your solenoid to feed the deck. This can cause a great amount of headache and will greatly be affected by your electrical back ground and the installers ingenuity. If cut wires or fried solenoid has occurred, you can set the solenoid using vice grips to your desired defrost setting and unplug the connection to inhibit it from moving directing steady flow to your windsheild.