I have a coleman electric mobile home furnace and air conditioning unit. If the fan motor is in the automatic position the fan cycles on and off with cooling on. If it is in the heating position the fan will not come on but will come on in manual. I installed a new thermostat but still done the same thing. What can possibly be wrong.

Sounds like you probably have a sequencer that controls the blower going bad. Look on the back of the blower door for a wiring diagram. If you have an electric meter and know how to use it track down the blower circuit to the fan heat blower control.These are usually piggy backed on to one of the heat element sequencers.