i recently replaced my radio and now the heater will not get hot

So does the blower motor run? Is there air blowing out of the vents, just getting cold air? The hot air comes from the water in the engine. Radio and hot water from the engine, two totally separate systems. If your blower motor is not running, it's a possibility during the installation of the stereo, there was something shorted out. Check your fusing, make sure the blower motor fuse is ok.

Probly just knocked the wire / cable out of the HVAC / A/C control unit when installing the new radio. If I remeber correctly one sit right above another. I would pull the radio and look for the obvious first.

The blower is working just not giving me heat. I had tried the heater after we put the radio in and it worked, but approximately 7 days later the heater went cold. There is water..........could it be the heater core?? Is this hard to replace?? How do I do this?