Use a meter and check for a short to ground. If shorted to ground the resistance between that circuit and the Batt Neg will be shown in Ohms. In a correctly working system it should be O.L. (Out of Limit) or on a very cheap meter it may say 10,000 ohms. If it shows 100 ohms or .5 ohms for example then there is a short to ground. The most common cause is a wire that has rubbed through it's insulation and the copper is now touching a metal surface. This is where a wiring manual comes in handy.