trailer lights

With a meter and a pin out diagram. Alternatively, a 12V battery, 12 Volt lamp and a pin out diagram.

Check the ground wire on the trailer.

All lights coming usually means a bad ground betweeen the vehicle and the trailer. Trailers do not ground well through the hitch. Use a separate ground wire.between chassis of the vehicle and the trailer chassis

You could test for reversed polarity, your trailers hook-up wires may be reversed they could be cross wired to ground, i would get a seperately powered coninuity tester & test the system without power - to ground and each bulb. if the ground side is touching the trailer chassis you need to check the + positve wire to each socket they must be seperate and properly wired to the socket from the plug. ~>

(if the system is isolated to it's wires) then you should check for shorts between both your positive conductors at the trailer (Plug) and possibly your hook-up outlet ~ for this you could use the 12VDC tester they sell at the auto or 99=E7 store. make sure you get the correct light/signal output at the vehicles socket, then break it down to your trailers plug/wiring & to the sockets. {you could leave the bulbs in and just see which ones come on when you connect each wire to a 12vdc source at the plug and then wire them back in to their corresponding contact on the plug} having made sure your socket is properly wired of course. Left & Right signals isolated & 2 seperate Stop contacts. if I'm not mistaken, I fixed one on my block not too long ago };-)

Roy

Is this a new problem or is it the first time you have hooked a trailer to the vehicle? If it is a first time and you have a vehicle that uses a the same wire for brake and turn signal you will need an adapter that is available at any parts store.