Is this a newly installed unit? The problem sounds like the sleeve is improperly installed. The sleeve should slope approximately 1/4 inch downward from inside to outside. This way the water will run to the back of the pan where it will be picked up by the slinger fan, or drain to the outside. Most likely this is the cause, not the styrofoam pan. If you need further info, you can contact me at greg.mcnary(at)airdistributors.com (replace (at) with @) and I can get you detailed installation instructions.
Air Distributors Company, Inc.
Distributors of GE Zoneline PTAC Units
andMcQuay PTAC Units