2008 Camry rear defrost light blinks at startup

Yes, the switch is the next thing to check.

https://www.google.com/search?q=2008+pontiac+grand+prix+rear+defogger+not+working

https://fuse-box.info/pontiac/pontiac-grand-prix-2004-2008-fuses-and-relays

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Have the AC system checked for a low refrigerant charge.

may be a factory setting do u use all of the controls on the panel for air, etc?

FSm covers that
alldata.com ,log in and read like we do.?
remove skins, the unscrew bolts (obvious)

All the functions that you are talking about are controlled by the BCM. This is an expensive auxiliary computer in the cabin.

The only way that carbon can get in the tank is from the canister. If they didn't run fault codes and tell you what they found and what was replaced I would guess that they were not trained enough in their job. Fuel filters take out carbon so if they are right then the filter needs replacing. I would go some where else and have the fault codes read

The maintenance required light lets you know its time for an oil change. Once the oil is changed the light can be reset. The light will usually blink for a few seconds as you approach the 5,000 mile mark and then stay on steady as the 5,000 mile mark is reached or surpassed.

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

REPLACE BUSTED FUSES

Front Open The Hood And View Back Of Headlight Turn And Then Pull Light.
Rear Open Trunk And Remove Carpet From Behind Light Now You Can View The Bulb Housing Turn And Pull.