an old unit like that may not have accomodation for fan in heat mode. it may depend upon the thermostat turning it on. all thermostats have accomodation to force the fan on when there is a call for heat. examine your thermostat and look for a switch with two positions, either 'g' or 'e'. place the switch in the 'e' position. with some older model thermostats, it requires a wiring change to accomplish this. has the thermostat been changed lately ?
if there are sequencers, a repair needs to be made. in the mean time, the above advice will ensure you have air blowing across the heat strips.