Basic continuity (ohms) testing will show up resistance between two points (either meter cable), the resistance for example of a good cable will be virtually 0. If the cable is worn by say salt water it will show a high resistance. When doing continuity checks on cars its always recommended to disconnect the battery negative and earth the meter to a chassis earth. Example, with the battery - terminal disconnected, put the positive end of the meter on the engine block and the negative on the chassis. A good earth will show virtually 0. Before you do each earth test though, touch the meter cables together, this will zero the meter. When testing car voltages, no voltage on the car will exceed the maximum output of the alternator (usually about 14 volts with the engine running), so set the meter to 20 volts dc to test. Always connect the positive (live feed) of a circuit first and always earth to the chassis not the battery negative. As for measuring current, you have to be very careful. Most multimeters will only go up to 10 amps, anymore and they will blow their internal fuse! A clamp meter however will measure very high currents by measuring the magnetic field by the clamp when its round a cable. They are the best and usually most expensive multimeters. If you are 100% certain the current does not exceed the meters maximum you can test it by putting the meter in line with the circuit (in series). Example. To test the current of a 5 amp bulb, set the meter to amps. Connect the positive of the meter to the battery positive, the negative of the meter to the centre pin of the bulb (can be either side but electrically correct is good practice), the side of the bulb is the negative which goes back to the battery negative. The meter would show you the bulbs current draw of 5 amps. If its higher, the bulb is failing. Still wouldn't recommend doing current tests though only with a clamp meter. Most electical car faults are found from continuity checks, bad earths through corrosion or loose connections. Broken cables, jammed relays etc. Their are some excellent visual lessons on using meters on utube under ASKTHEMECHANIC.