Get the truck warmed up, turn on the A/C, put your nose up to a vent, and take a big whiff. If you smell the antifreeze strongly through that vent, you have a bad heater core. Also get a good flashlight and take a good long look at the two heater hoses where they attach to the heater core going into the firewall. That might be your leak. Prepare for a hell job. It has been discussed here many times.