That small rubber hose on the passenger side of a 1990 Cavalier's firewall, near the AC lines, is most likely a
vacuum line. It's part of the vehicle's vacuum system and is used to control various components, such as the HVAC system (heater, AC, and blower motor controls).
While it's impossible to be 100% certain without seeing the specific vehicle, the location strongly suggests a vacuum line related to climate control. It's possible it could also be a very small drain line, but vacuum lines are far more common in that area.
