Hi, there is a tsb(technical service bulletin) that talks about your concern. This may be the problem.

#05-01-38-016B: Water Leaking on Front Passenger Side Floor of Vehicle, Water Blowing Out of Vents (Seal Heater Core Cover) - (Aug 16, 2006)

This bulletin is being revised to provide the part number of the screw that is required for the Cobalt and Pursuit models. Please discard Corporate Bulletin Number 05-01-38-016A (Section 01 -- HVAC).

Some customers may comment on water leaking on the front passenger side floor of the vehicle. This leak would occur under high ambient temperatures and high humidity situations when the customer has the air conditioning on high. Leaks will appear to be coming from the HVAC module where the heater core cover and lower case come together. Also, some customers may comment on water droplets blowing out of the vents when the A/C is on high.

Cause

This condition may be due to water condensing in the HVAC case that adheres to the upper surface in the tongue and groove area of the lower case and then leaking out.

Correction

Install the foam gasket between the heater core cover and the lower HVAC case at the location of the leak using the procedure listed below.

1. Remove the right side insulator/closeout panel retainers and the panel, if equipped.
   
   
   
2. Remove the 5 screws (1) (HHR) or break the 4 welds (2) (Cobalt/Pursuit) from the heater core cover at the specified locations.
   
   
   
3. Carefully pry apart the heater core cover and the lower HVAC case to achieve a 12.7 mm (0.5 in) gap.
   
   
   
   
   
   
4. Install the foam gasket, P/N 15878211, using a flat rigid tool (feeler gauge shown) into the sump area (1) of the heater core cover.
5. Reassemble the case using the 5 screws that were removed (HHR) or with new screws (Cobalt/Pursuit), P/N 11609829. For Cobalt/Pursuit, add an additional screw (2) at rear of sump area.
6. Install the right side insulator/closeout panel and retainers, if equipped
7. Let me know if i can help you further. Please click on accept. Thanks ERic

The ac drain tube is on passenger side of the firewall, near the bottom behind engine, its tough to see from the top sometimes, it usually has a short rubber hose on it pointing to the ground for drainage. The most likely cause of your concern is not a plugged drain tube, but the tsb i previously gave you. Good Luck, and let me know if i can help you further, thanksEric