Just in case someone wanted some kind of reference to the amount of noise I heard, here is my subjective db values based on the vol at 50%. I have the Boom Audio Kit with the upgraded speakers, amp, and bass booster ports installed on a 2011 Ultra Limited.
Aux port from GPS without the NIM installed - Base line reference will call 0db. Actually it pretty much was 0db. I didn't hear any noise at all at 50% volume.
Aux port with NIM installed, GPS not plugged in - noise level up to about +18db. Frequency of the noise is probably a composite of frequencies found in the 1kHz - 14kHz range.
Aux port with NIM installed, GPS plugged in and on - noise level up to about +12db. Couldn't be heard riding, but could be heard when stopped at a light.
Aux port with NIM installed, GPS not plugged in, ground loop isolator installed - noise level down to about +3db.
Aux port with NIM installed, GPS plugged in, ground loop isolator installed - noise level down to about +1db.
Granted, most people can only hear a change in volume in 3db increments, but I've been blessed and cursed with ears that heard crap over 18kHz like it was stuck in my ear and apparently can notice 1db changes when its in the right frequency range.
Anyhow, again, this is all subjective measurements, but should give you an idea of how it came across to me.