Only advice one should get should be from your cars owners manual and/or dealer. Each car is different. Alot of cars made today are not designed to have trailer lighting attached at all or could with additional module. You could cause damage to your cars electrical or computer systems. When testing one should use a multimeter. If car is not set up for it from factory, the days of using test lights and just hooking up lights are over. Test light could even cause damage.
Here's statement from site that will back up what I am saying:
Standard & Power Taillight Converters
Today's vehicles have more electronic features than ever before. New advancements include on board computers, cruise control, ABS brakes, air bags - systems that require more electrical power and more protection.
When you connect a trailer to a modern vehicle with sophisticated electronics you need to be aware that these features can also lead to more potential problems. Failing to use the proper wiring system for your tow vehicle can have many unfortunate consequences. An inadequate wiring system can cause an overload, resulting in blown fuses and potentially damaging sensitive, and expensive, electronic devices on your tow vehicle.
The following information will help you select the appropriate wiring system for your towing needs. As you will see, that selection is dependent not only on the tow vehicle, but also on the trailer that is being towed. The information in this online catalog is believed to be correct. However: Car and truck manufactures may change their system at any time. Be sure to carefully review any information in your Owner's Manual regarding trailer wiring.
Here's link to whole page:
https://www.bageco.com/info/wiringinfo.htm
