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Honestly? The dealer. The 04s are a different computer than previous years. Gonna be hard to find one used, but might be possible.
How do I know? I fried mine. If memory serves cost on the PCM is under $300, then by time you have the dealer flash the VIN and the program you'll spend about $550 if you get employee cost on the PCM.
Just buy one of the scanning/tuning packages like EFILive or HP Tuners. Then you can download your custom tune to your laptop, upload the stock tune for the dealer and then restore your custom tune when you get hte car back. It's a heck of a lot easier - ~30 seconds to upload or download a tune. Pulling the PCM physically is a bit of a pain. Swapping a tune is a brezze. Plus, you'll have the capability to scan and tune your PCM.
Heh, I just take mine in with custom tune. Never had an issue. The dealer knows it's been tuned too. They even did the column lock recall without affecting my tune.
I had them include of the service contract not to touch the PCM, it had a custom program. Touch it and you will pay for towing and a new cumputer tune.