The heater core is simply a little radiator - nothing electrical there.
There is, however, an electrical control valve that regulates the flow of
hot coolant from the engine through the core. I suspect that you may have
disturbed the wires that operate that control valve. If you were working on
the stereo, you were not far from these wires. On the back of the
temperature control knob (in the middle under the stereo), you should find
the connections - make sure that these are all in place. While you're there,
check the vacuum lines on the defrost/vent/heater selector (just to the
right) and make sure that they are not broken, kinked or missing.

Good luck.

Wes