That's the connector that the light is located on TMK. On our three wire setups one should be F or field to turn on the alternator to charge, S for Sense which is where you add the diode to fool the alt into thinking the power is lower than it actually sees, and L for Light...this is likely your problem wire/connection. IIRC when the regulator goes out, it grounds this wire. Or I'm just thinking that because it's Monday, and intelligence and Monday's aren't exactly best friends.