Hey 370zFTW,
One item to think about is the antenna in our car, it may be powered, which means it may require +12V to activate, in this case the blue harness wire will be the "Power Antenna" lead, in the old days folks used it to activate the power up/down antennas, now they're used to power the AM/FM antennas, some are diversity (two) or single, and some of them require +12V to work optimally.
As Mitco pointed out, the Metra harness should address that with a connection to the antenna if it is powered. If your AM/FM reception isn't as good as before the radio swap I'd look at the powered antenna possibility. If it doesn't matter then never mind!
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