Because lighting up the brake lights of both the car and the trailer pulls too much power. If the problem is a specific trailer then your trailer may be mis-wired. You may also have a mis-wired trailer wire adapter.
#1 reason is always bad GROUND. Now you really weren't all that specific about which plug. Trailer plug on the vehicle or actually on the trailer? If it's the plug on the vehicle, I would strongly suspect bad ground or installation was backwards. If on the trailer, merely backwards installation. In any case, get better trailer light performance by running separate ground wires to each individual light socket on the trailer and connect them to the ground wire at the plug so they will be grounded to the vehicle at time of connection. I do this with all of my trailer lighting and never have issues.
The ground wire for trailer lights is typically white.
most times its just the bulb or socket or open wire are both tail lights not lighting? fuse, bad bulbs, socket/s, loose ground or open in wires, headlight switch or brake pedal switch
To wire a plug socket correctly, first turn off the power to the socket. Then, strip the insulation from the wires and connect the live wire to the brass terminal, the neutral wire to the silver terminal, and the ground wire to the green terminal. Finally, secure the wires in place and screw the socket cover back on.
The hot wire's cover is smooth and connected to what I call the "button" at the bottom of the socket. The neutral wire's cover has ridges and is connected to the screw shell of the socket.
To wire a lamp socket correctly, first turn off the power. Then, connect the neutral wire to the silver screw and the hot wire to the gold screw on the socket. Make sure the wires are securely attached and the socket is properly grounded. Finally, test the lamp to ensure it is working safely.
I'm guessing you're talking about the plug for the lights not a battery on a travel trailer. You have a short in the wiring somewhere. Check the trailer harness for a pinched wire of even a bad socket. You will probably blow the fuse in the vehicle if you plug it in and leave it. Use a battery to isolate the problem while testing.
In a household circuit, with a "hot" conductor insulated black and a white neutral, the black wire should connect to the center terminal of the socket. The outside part of the socket usually has a brass screw (for the black wire) and a nickel screw (for the white wire).
The easiest way to answer this would be to remove a taillight and look at the wires on the back of the "backup" light bulb socket. This is also a good place to tap into the wire, provided that you are able to route the new wire so as not to rub on any sharp surfaces. For example, if you are trying to tie the backup light circuit into an aftermarket stereo fitted with a reverse camera, you will need to be able to route the wire all the way to the cab of the vehicle without leaving it exposed to damage from rubbing, weather, and other possibilities. If you are trying to tie the circuit into a trailer lighting system, the same rules apply but with a lot less wire (as the wire only needs to go a few feet to the trailer hitch). Same applies for a backup alarm.Hope that helps.JB
To wire a light bulb socket, first turn off the power. Connect the black wire to the brass screw and the white wire to the silver screw. Make sure the wires are securely attached and the socket is properly grounded. Test the connection before turning the power back on.
Chances are very good that you have a bad ground wire on your trailer or on the connector. Check the connector for rust or rot, clean the pins and use some grease to prevent future problems. If the problem isn't at the connector, you may have a bad ground on the trailer which could be anything from an ungrounded light socket to a bad ground wire. Hope this helps