If or when doing this on a denso alt. remember the LED is polarity sensitive. The light circuit on the denso works with an ignition hot feed to the light, and the negative side goes to the L terminal.
When alternator is outputting, L terminal is open. If alternator faults, or during ign on start L terminal contact is closed to ground completing light circuit.
I used a #50 machinests drill to make the light hole as small as i could to hide it. Then i used a black sharpie to "paint" the edges of the drill hole. JB weld was used to glue it in. The positive light lead was soldered to the 12V hot wire for the gage. Black goes to L terminal on Alt .