ok, 5 p worth coming up. Could be anywhere if it's a bad earth. Try wiggling the connectors at the rr of the alternator, does anything fall off? Don't do this whilst it's running. If its over say 5 years old my experience is the thing will need replacing, that's the rate I seem to get through mine. check your battery health, is it running 13.?V when running and 12 or more when not. Any issues with starting? Your alt may be on the way out. Have you made anhy mods recently, typically it's user error.