Coolant pump on storage tank operation

I am hoping someone can help me with this issue. I replaced the coolant pump that is attached to the storage tank (front left) and I know that pump works as I bench tested it. For whatever reason, when I park the car after use that pump is not coming on and moving coolant into the tank. I am hoping someone knows what triggers that pump to turn on. I know the ECU turns it on but there must be a sensor etc. that tells the ECU to send power to that pump. I just want to know the sequence of events that turns that pump on. Thanks to all!

 

Thanks so much..... I just read this and want to go out to the garage but will wait until the morning. Thanks again!

 

I've watched this guy before. I think he must be in college by now. Thank you.

 

UPDATE: I wanted to put an update her for all to read. Frist I wanted to thank everyone for their comments and help. This is without a doubt the best site with the best people on it! Thank you!
I don't know why I thought of it but I decided to disconnect the 12v battery overnight. When I connected the battery back up this morning the car was working perfectly. The auxiliary pump was working as it did and everything seemed fine. I drove it around and each time I stopped and shut the car off, the pump came on as it always did. I am hoping this issue is an aberration and not something worse. In closing, thanks to everyone.