I have a Heil heat/air gas pack(forced air) unit. It's about 20 years old, but it's been a great one. When I flip the t-stat to HEAT and AUTO I hear the unit outside firing up and I can see heat rising out of the vent, but the blower won't come on to push heat inside my house. When I turn the t-stat onto FAN(and the other switch onto HEAT), everything works fine, and heat is blown into house. Can't be the blower because it works on FAN. What do you think?

Often blowers have a higher speed for cooling and fan on. They use a different relay or limit switch to energize a different winding for a slower speed for heat. So check to see if the limit switch is closing shortly after the gas comes on. Also check to see if any relay for the blower is working. With an older system you may have a separate component easy to trouble shoot and cheap to replace. Newer systems have everything on a control board. $$$$

You got to decide what controls the heat blower speed it could be a fan limit control, or a PC board. 20 years old huh, the last time it was serviced? Fer safety sake people if you don't know how to inspect a heat exchanger, call a pro. You have know idea how many Americans get CO poisoning each year, and a day in a hyperbolic chamber is very expensive, if your that lucky.