I'll throw in what I know to check, which isn't much.
I'm not sure about the line you refer to, but I have been told there is a filter on a line somewhere. If your accumulator is beginning to rot through, then particles can clog this filter, or at least so I've been told.
Get the pressure checked.
Does the system cycle when it's blowing the warmer air.
If the AC system is not cycling then the air will be warmer or the same as regular vents.
When it's blowing warm air do you have to park it for a while before it blows warm again, or will switching to panel for a minute then back again re-engage it?
Other than this, I don't know what to suggest.
