If the dash vents are blowing cooler on the driver side than they are on the passenger side, the problem is with the heater core. It is partially blocked or plugged up somehow.
I know this because I just went through it - twice.
You have two options (assuming the core isn't leaking).
1. Try flushing the heater core. To do this you'll have to remove both coolant lines from the heater core at the firewall. You could remove the lines at the waterpump as well, but you'll end losing a good portion of your coolant. If you're going to flush the coolant anyway as part of the job, then this may be the better option. If you know your coolant is good, then pulling the lines at the firewall will minimize coolant loss to maybe a cupful.
With the coolant lines removed, get yourself some surrogate lines to attach to the heater core. You'll only need a couple feet for each. Dump some CLR (or muriatic acid) into the lines and let it sit for a bit, maybe 30 minutes or so. After you've let the CLR do its thing, hook up a garden hose to the OUTFEED line, not the intake line, and give it a good blast of water. Running the water backwards through the heater core should flush out any debris that's lodges in there.
Lastly, hook the coolant lines back up and start up the truck and let it run for a bit. Make sure you burp all the air out of the system before closing up the rad cap.
Note - this is note a guaranteed repair. It may or may not work depending on how plugged up the heater core is. It is, however, a good inexpensive first step to fixing the problem.
Option 2 - Replace the heater core
Okay, this is a pain in the hind end. I'm not going to go through all the steps to doing it, there are a couple good youtube videos out there that'll show you most of the steps. Suffice it to say, if you've never done it before, or are a novice with the tools, plan on a full day to do the job, maybe even a second.
My heater core first failed back in early December. I tried flushing it, but it only helped marginally. I still didn't have good heat. I then decided to replace it. It took me all day Saturday and a little bit into Sunday to finish it. My heat instantly returned.
Now fast forward to about 2 -1/2 weeks ago. Here in the Toronto area we had about three solid days of -25C temps. It turned out my coolant wasn't up to snuff and it froze solid, at least in the heater core. The truck sat for about a day and a half. I went out that Sunday afternoon to run an errand. No heat! None whatsoever. After about 3 hours of the truck running, it finally came back, but with the same problem again - cool on the driver's side and warm (not hot) on the passenger. [email protected]
*K!!! I was pissed.
I tried flushing it at first but it didn't work. So last week, I changed the heater core again. Fortunately, this time because I kinda knew what I was doing, it took me a little under three hours start to finish.
After I removed the heater core, I took it into my kitchen and ran hot water through it (both directions). I could feel the temperature change on the upper half of the core (got hotter), but the bottom half of the core stayed cold. And when you look at how the heater core is positioned in the air box assembly, the bottom half of the heater core is the side that feeds the driver side.