I have a 2003 Honda Civic ex, the a/c works great but i'm unable to change to heat?

I have a 2003 Honda Civic ex, the a/c works great but i'm unable to change to heat?

Take the knob off and see if it is not broken inside. Those knobs are in stock at the Honda dealer.

I was told it may be the resister, but can't find where it is located