Never heard of the water deflector but the cabin air filter is located behind the glove box and usually requires removal of the glove box to replace.

I took my 04 Comp G apart last night for the same issue (the water deflector fell off and way laying on the filter).  The air filter is NOT behind the glove box.  It's under the cowl.  Here's some info:

Date: February 25, 2005

TECHNICAL

Subject:

Water Leak On Front Passenger Floor (Install Water Deflector)

Models:
2005 Buick LaCrosse, Allure (Canada Only)
2004-2005 Pontiac Grand Prix

Supercede:

This bulletin is being revised to update the models, model years and Correction information. Please discard Corporate Bulletin Number 03-08-57-006A (Section 08 - Body And Accessories)

Condition

Some customers may comment on water leaking onto the front passenger carpet.

Cause

Water may be getting past the rubber water deflector located under the air inlet grille panel.

During diagnosis, the technician may note that the passenger compartment air filter is wet.

Correction

Install the new plenum water deflector. Follow the procedure below:

1. Position the windshield wipers in the up position.

2. Open the hood.

3. Remove the hood seal.

4. Remove the washer hose from the right air inlet grille panel.

5. Remove the push pin that retains the air inlet grille panel.

6. Remove the air inlet grille panel.


Important: If Corporate Bulletin Number 03-08-57-006A was performed, it will be necessary to remove the strip of foam that was installed on the front edge of the cowl.

7. Discard the rubber water deflector.

8. Install the new designed plenum water deflector.

9. Install the air inlet grille panel.

10. Install the push pins.

11. Install the washer hose.

12. Install the hood seal.

13. Close the hood.

14. Return the windshield wipers to the park position.
Parts Information

Parts are expected to be available from GMSPO on 3/2/05.

_OR__

Remove the foam/rubber seal.Clean both the seal and metal where its fits onto.Prime and ureathane back on.They fall off and water goes into the vent.Easy fast fix,design flaw. \

thank you i got this fixed at the dealer and was aware of this bulletin they tried to charge 135$ crazy the part is like 7$ dollars so any way they wanted 40 for the cabin airfilter i politely said no and got one for 17$ and took the 5 minute job apart and put my own filter in in the end i paid only fortysixdollars for the dealer work and 17$ and put the filter in myself and wish i had the dealer part it would have only cost me 24dollrs. not 63dollars