Heater core?

If the truck came from the factory with air conditioning, there will be a drain pan under the heater core to collect the water that condenses on the evaporator. If the heater core is leaking, you'll find the coolant dripping from that drain onto the ground behind and left of the passenger front tire. Here is a guide that will help walk you through the steps of changing it with diagrams below to show you how on your truck.

https://www.2carpros.com/articles/replace-heater-core

Check out the diagrams (Below). Please let us know if you need anything else to get the problem fixed.