I have a 93 Seville with 27,000 miles.
My radiator sprung a leak 2 weeks ago. I removed it and had the side tank re-crimped. I then re-installed with new fluid (green, 50/50) and all is well. Oh, added BarsLeak pellets too.

Since I only inherited this car 3 months ago, I'm trying to get used to its quirks.

However, its now in the middle of a cold snap and I notice that when on "heat", the front footwell vents blow hot (very hot) on the passenger side, and cool at best on the driver's side.

At first I thought maybe I had an air bubble in the system... but that doesn't make sense. Then I thought the heater core was plugged... but how could I get hot air on the passenger side?

The defroster does the same.... cool on the driver's side, hot on the passenger side.

So.... It's gotta be a "door" stuck somewhere? Any ideas? How do I access the "door" that should swing open or shut for heat? I can pull the center console if its under the dash in that area.... or is it somewhere else??? I don't have the FSM. But I have wrenches!

Where's the best place to look, access the vacuum hoses and door?

Thanks in advance.........

Start by checking the operation of all the air routing functions: defrost, dash vents, dash vents/floor, etc. If the doors and blower operate correctly (equal air from here and there......) then there's a coolant problem, BUT, that's unlikely if, as you post, the passenger side blows hot. Run the blower on high while shifting modes.

Is a duct or air hose disconnected or crimped?

If sediment settles in the heater core the lower portion does not get hot coolant flow, thus the air passing through can get stratified (hot on the passengers side and cold on the drivers). Try flushing it.