Visually inspect the wiring, look to see if it's been chaffed etc allowing the actual wire to come into contact with metal.

This is if it's doing it when the indicators arn't applied, if it's when the indicators are applied, check the clusters for water ingress, also check the repeaters in addition make sure these have not be interferred with as this is a common way to disable some alarm systems if a theft attempt has been made, not going to go into detail on here for obvious reasons.

Martin