I have the trailer hookup on the back of my truck and over the years I haven't always remembered to close the little rubber cover on it. I am putting new lights on my Jon boat trailer and am concerned I need to replace the hookup on my truck as well. Can anyone simply explain how to use a multimeter to check the light hookup on my truck?

Each one of the pins on the connector, be it 4, 5 or 7, has a specific purpose. The right way to check things is to set the meter to DC voltage, connect the black lead to a vehicle ground. Test each function with the red lead. When the lights are on you should see 12v or more on the pin for the lights(brown). When the signal is on, you should see 12v 0v 12v 0v on the pin as the signal turns on and off(yellow left, green right). When the brakes are applied you should see 12v to the same pins as the signals(yellow and green) but constant and at the same time. If you have a 5 pin, the you check that with the vehicle in reverse(blue).