There is a wiring loom that comes with the tach which needs to replace a loom on earlier bikes like your 03 - did you install that replacement loom? I just installed a tach on my America but it didn't need the loom that comes supplied with the tach because it's an 09.

Cheers, Richard
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09 America, Staintune Pipes, K&N, Breathe, Hagon Nitros, AI & O2 removed, tune 20184 (modified), MTX-L a/f gauge