Hi, Welcome to Just Answer, I am Clayton. I can help you with this.
Are you replacing the Cab heater core, or the bunk. I also need the last 6 of your VIN
I have uploaded some instructions in PDF form. Click the blue letters below.
Instead of draining the coolant, I usually take hose clamp pliers and clamp them off, saves a little time.
You will have to cut a few zipties that is holding the dash harness on the plastic cover, the harness is kind of a pain, but the overall job is not bad.
CLICK HERE FOR INSTRUCTIONS