The standard electrical connection on new vehicles is a 7 pin blade connector. The wiring on this trailer was in tact but it had a 4 pin connector and it had an extra connector for a rear hitch. So, I had to sort through the electrical and determine which wires went to what, then connect the new plug.
Trailer lights are very simple. Virtually every trailer I've restored has had the wiring jerry rigged because people make it harder than it should be. There are 2 lights on the back of a trailer. Each light either has 2 bulbs or 1 bulb with 2 elements (2 separate ways for it to light up). Every light/component requires a positive and negative connection.
On the left of your trailer, you have a turn signal (this light is also used for the brake light) and a tail light (the light that comes on with your head lights). The same for the right side. For a total of 3 positive wires.
1- The left turn signal from your car needs to connect to the left signal light on the trailer.
2- The same for the right.
3- The tail lights from your car need to connect to the tail lights on both sides of the trailer. This same wire runs to the running lights which are on the sides of the trailer.
The negative wire from the car connects to the frame and skin of the trailer. So when a light fixture is screwed to the skin, it automatically connects to the negative. This provides positive and negative to each light.
Here is a picture of a typical tail light. It has 2 wires coming out of it. 1 wire is for the turn/brake lights and 1 is for the tail lights. The internal wiring connects the negative to the skin of the trailer simply when you screw it to the trailer.
To test your lights, you can use a simple battery charger. Connect the negative to the trailer frame and then connect the positive to one wire. Go to the back of the trailer and see which light is on. If it's the right signal, label the wire 'r-turn' and move on to the next wire.
If you're not getting any lights to light up with the charger, you can use a simple circuit tester where the lights are to see if electricity is coming through. Connect the alligator clip to the aluminum skin and use the sharp point on the tester to make contact with the positive wiring connection. For instance, if you don't know if the bulb is working or burnt, pull the bulb out and touch the tester to where the bulb would make contact. The circuit tester will light up if it's getting current and you'll know the bulb is burnt.
Below is a wiring diagram for a 7 pin connector. Every light/component requires a positive and negative connection. Under normal circumstances, current will feed to one side of a bulb and the other side would connect to the frame or aluminum skin of the trailer. The brake lights use the same bulb as the turn signals. So, there is...
1 wire to the left signal/brake light (yellow/5).
1 wire to the right signal/brake light (green/6).
1 wire to both tail lights and running lights (the little lights on the sides of the trailer, lit when head lights are on) (brown/3).
The connector has provision for electric brakes (blue/2).
1 wire would go the brakes and the negative side of the brake system will connect to the frame.
1 wire would go to the battery (black/4). This would keep it charged. If you don't have a battery it can connect directly to the 12 volt circuit on the trailer used for interior lights and any other 12 volt appliance.
The center connection is an auxiliary connection you can use for whatever you want. Many people will connect it to a back up light system (orange/no number).
And finally, one wire is connected to the frame of the trailer (white/1). Make sure there is a wire from the frame to the skin to provide negative to the skin.
If your rig is wired correctly, everything will work as desired.

Hope these tip's are helpful. They are offered without warrantee for entertainment purposes only.
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