Diming gauge bulb
 

97 elecrta glide classic, when using the turn signal, only the bulb on the speedometer will dim when using the turn signal while flashing. Any ideas why?

Bulbs get their power from the same source wire, how about the neutral light, dose it dim when your run the turn signal? How is the state of charge in your battery, is your stator putting out enough power? Is the Voltage Regular operating correctly?

The only bulb dimming is the one in the speedometer, all other bulbs including the neural light do not dim

Maybe speedometer bulb is going south, or socket corrosion.

Is the bulb replaceable or do I have to get the bulb and socket together?

Looks like it is replaceable. Check Ronnie's parts fiche for the details. Go-to the instrument panel page.