I have BDs on all our autos. basically the work like this. Temp fall = blow. when the enter wash sign ,or stop sign goes off ,wait then blow.
I use a thermostat with off set so that I can set if the temp hovers around the turn on temp ,it will not be blowing every 3 min.
I use the enter light outside the bay ,so that if a customer goes throughthe gattling guns ,but do not make it to the stop area in time it will blow,and not freeze the UC. If the stop comes on my system will wait until the wash is over,then blow.
I have in the past used a relay,and set the plc for constant weep. That way if the weep goes off the relay makes ,and triggers the blowdown system.
There is no reason if you have a spare output on your weepmizer,that you could use it to trigger your BD system.