To save weight and cost the metal of a car body is used as a return path to battery negative - You will notice a short fat black cable from the battery Neg terminal to a nearby place on the car's bodywork.
Take the Horn for instance - just one wire from the horn switch will go to the horn and a short wire will connect to the body - either directly or indirectly - to complete the circuit.
A bad earth is a loose, dirty, missing or coroded connection to the bodywork. You need to find the bad earth - clean it up - fit a star washer perhaps to cut through the paintwork and tighten the connection.
Much will depend on the failing component - i.e. a failing rear light will not have it's associated earth point at the front of the car.
There are people on here (nice people - wise people) who maintain that some "bad earths" on cars can mysterously make things operate that shouldn't be operating i.e. Operate the Headlights and the wipers suddenly start up - bad earth they might say. Frankly I find this perposterous - when challenged they are unable to explain how this is happening - an open circuit (bad earth) can't possibly energise a completely different circuit - all a bad earth can do is stop something working. (as far as I know)
Good luck...
Peccavi
November 2009
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix cars.