Test a seven-way round pin trailer plug by using a standard probe-type electrical tester, or a volt meter, that is available at most auto parts stores. A second person must sit inside of the truck and run through each of the functions while the tester is contacted with the correct pin. Probe-type testers illuminate and volt meters indicate the correct current flow if the circuit is good.
If the configuration of the wiring for the seven-way round pin trailer plug is not known and a wiring diagram is not available, it may be necessary to test all of the pins for each function until the correct one can be identified. If none of the pin's connections register on the tester, check for corrosion at the contact points followed by inspection of the system ground. If those items are good, the wires must be inspected from the trailer plug to the forward connection point to search for broken or exposed wiring.
If the tester shows a good connection but the equipment on the circuit is still not working, test the individual elements. For tail lights, brake lights or running lights, remove the lens cover and examine the bulbs to ensure they are not burned out.