I have a Hunter thermostat. Model 42999B. I can get ac but no heat. Can you help me with wiring it correctly?

Many times I see white for heat, and may connect to the W.

Depends on the type of wire used to wire the stat... ex. 18/4 or 18/5 or maybe even 18/7. As a rule though Red to Red, White to White takes care of heat only.