In order to get the coordinates for this cache, you must complete the nearby Historic Helena #1 Adventure Lab. Logging all 5 locations will reveal the coordinates to the final, Bonus Cache. If you go a few yards East, you will find another historic sign for the Carriage House.
In order to participate in Adventure Labs, you need the app. You can find information here: https://www.geocaching.com/sites/adventure-lab/en/
The following link is for the Adventure Lab:
https://labs.geocaching.com/goto/HistoricHelena1