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Cool, another 98 in San Diego! I also have a '98, and I'll be moving to San Diego. If you want to get together and figure this tuning thing out, let me know. I already have the software & hardware to tune our cars.
To expand on the info above, the 97-98's had the regulator in the engine bay, and the second line is a return line. After '98, they placed the regulator next to the fuel filter, which is in the tunnel above the torque tube. I believe in '04, they started placing the fuel filter and regulator in the fuel tank to reduce emissions. More info can be found in the C5 General Sticky.