Trailblazer LT 4.3L V6

• Automatic transmission
• 64,863 miles

The problem first started with the external display showing 50 degrees if it was higher than 80 degrees outside (this was last summer). First it was intermittent, and then consistent. Then when winter came, the heater wouldn't work altogether, and after about a week or two, just "started working" again. Now that it's getting warmer, external temperature only displays correct temperature if below 50 degrees, and even that is intermittent now. BUT the BIG PAIN IN THE ASS problem, which will escalate to a "#5" when it hits 80-90 degrees is that the heater runs ALL THE TIME on the passenger side, and in the back seat (have separate controls in back, but they must run off passenger settings). This problem happens whether you have the thermostat set to 60 (lowest setting) on passenger, driver, or both sides, and the heater will CONTINUE to run even when you have the Climate Control completely turned off! Air conditioner works fine on driver's side, and it's only "bothersome" right now if you have it set to vent and feet... vent blows cold, feet is HOT... This is ridiculous, and I can't afford to take it to the dealer to get fixed... they wanted to keep my car for a minimum of 2 days! Is there something simple that can be done to fix or check? Apparently this is a common problemm that I'm seeing posted in several places online... HELP! ROASTING in CALIFORNIA!!!