Can I salvage the center piece of the radiator air deflector that hooks onto the bottom of the radiator behind the bumper of my 2004 zts sedan? The clips for the side pieces of the air deflector are broken off and the center piece lost the rivets and fell off. I read that some people have used zip ties to reconnect it instead of the rivets. Is this safe? Should I remove the broken side pieces and replace them or can I go without them?

There isn't one , not necessary if you follow directions
Close the radiator drain ****.
Install the front air deflector. Refer to Front Air Deflector Replacement in Body Front End.
Lower the vehicle.
Important: Use a 50/50 mixture of DEX-COOL antifreeze and clean, drinkable water.
Slowly fill the cooling system with a 50/50 coolant mixture. Refer to Approximate Fluid Capacities in Maintenance and Lubrication.
Install the coolant pressure cap.
Start the engine.
Run the engine at 2,000-2,500 RPM until the engine reaches normal operating temperature.
Allow the engine to idle for 3 minutes.
Shut the engine OFF.
Allow the engine to cool.
Top off the coolant (1) as necessary.
Inspect the concentration of the engine coolant, using the J 26568 .
Rinse away any excess coolant from the engine and the engine compartment.

Raise the hood.
Lift the flap on the appropriate end of the radiator upper air deflector.
Separate the radiator side air baffle from the upper air deflector.
Remove the fastener retaining the upper edge of the front bumper fascia to the headlamp bracket.
Remove the two bolts retaining the headlamp assembly to the headlamp bracket.
Slide the headlamp assembly slightly inboard to release the outboard retaining tabs. Carefully pull the upper edge of the front bumper fascia away from the inboard edge of the headlamp assembly enough to aid in headlamp removal. Carefully remove the headlamp assembly from headlamp bracket.

Normally this piece of plastic is the low side air deflector...also diverts air to cooling system...You can try to re-attach to factory location...Yes, this part should be on the shelf at your Honda Dealer...Hope this helps.

The radiator is leaking?

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

If you have the new deflector already, go to your local auto parts store and get 8 - 10 plastic retainers. Pry the old ones open and pop them out. Put the new deflector in place and "pop" in the new retainers. Did mine in about 5 minutes. Don't get the retainers at the dealer. They are about 3 bucks a piece. At O'Reilley's they are 2 for a buck (I Think, It's been a while.) This is an easy fix that will really improve the looks, and resale value, of your CTS. Hope this helps.

it is a air deflector

Windshield molding? I'm assuming you are talking about the interior plastic pieces which are decorative to cover the bare metal frame of the vehicle?
If so the best and cheapest place is your local junk yard now possibly called vehicle parts recycling center. ;-)

Air Spoiler is the name of the deflector you are referring to under the front bumper, if you need another head on out to your local junk yard for a cheap replacement, if lucky you can find one in the same color.

Here are the steps:

1. Raise the hood.
2. Lift the flap on the appropriate end of the radiator upper air deflector.
3. Separate the radiator side air baffle from the upper air deflector.
4. Remove the fastener retaining the upper edge of the front bumper fascia to the headlamp bracket.
5. Remove the two bolts retaining the headlamp assembly to the headlamp bracket.
6. Slide the headlamp assembly slightly inboard to release the outboard retaining tabs.
7. Carefully pull the upper edge of the front bumper fascia away from the inboard edge of the headlamp assembly, just enough to aid in headlamp removal.
8. Carefully remove the headlamp assembly from the headlamp bracket.
   
9. Disconnect the electrical connectors from the headlamp bulbs.
10. Rotate the park/turn signal lamp socket counterclockwise to release.
11. Remove the headlamp assembly.

It is a lower air deflector.

if the numbers on the bottom of pan match .