A/C Condensation Problems can be the most frustrating and difficult to diagnose.  If it really is a condensation problem.

Problem # 1 details the simple fix for a genuine condensation problem.  Problem # 2 details the fix for a problem that "mimics" Problem # 1.  Problem # 3 is specific to 9C1 models.

Problem # 1:

Condensation coming out of the firewall a/c drain is running back inside the car.  Check that the rubber elbow is there.  If it is, do the a/c drip hose mod.  This mod will not only keep the draining condensation off of the ABS connector, but it will also prevent the possibility of the water being blown back into the interior of the car during normal driving.  If the rubber elbow is not there, get a new one and do the a/c drip hose mod anyway.  The instructions are located in the Impala SS Technical archives.

Problem # 2:

There have been a lot of reports of seam sealer being non-existent around the area under the wipers, especially the passenger side.    Remove the grille under the passenger side wiper and then remove the plastic plate under that.  Look down in the hole and see if Seam Sealer is present along all the corners.  Run your water hose to that area and turn it on a low flow.  The fun part is finding where it's coming in.  Remove the kick plate under the dash on the passenger side and pull back the carpet and black insulation stuff and look for where the water is coming from.  I had a leak about 6 inches up in the top RH corner.  Wherever you find the water coming in, force Seam Sealer in the area and run the water again to see if it is still coming in.  A real fun job!  I got my Seam Sealer at a NAPA store.  It usually can be found in the section where body repair stuff is located.  It is Tan in color and dries as hard as a rock.

Problem # 3:

You may have a big hole where a bunch of the 9C1 wiring went out through the firewall (I did).  My hole (2" ??) was right in the middle of the firewall on the passenger side and it was sealed with great gobs of that GE clear sealer.  When all the wiring was removed (along with the associated equipment) the clear sealer had a bunch of holes in it and a little water would get in there.  I had a piece of metal welded in to close the hole permanently.

Good Luck!