External fan to come on when AC unit comes on.
Here is an external, low-tech solution. Put a sail switch in front of the ac unit. Wire the switch in series with the secondary fan. A small open-ended box could enclose the switch and terminals. The airflow would turn it on.
If it is a central ac system you could put a thermodisc at one of the registers or at the plenum. Use a "for heating" model that switches on when it detects cold air.