over the last couple of years there have been several threads with problems concerning trailer wiring.
so here is an easy way to build your own power plug to use while running tests on the trailer instead of keeping the trailer hooked to a vehicle and running its battery down/blowing fuses you dont have spares for and banging your shins on the trailer tongue when trying to step over it.
go to the parts store and buy the female connector that matches your trailers male connector,this could be the flat four,the flat five,the round six pin for others,and the newer bigger seven pin connector.
while you are there pick up a short chunk of trailer wire,just make sure it has the proper color wires,ie,
green (right turn),yellow(left turn),brown(tail),white(ground) for the basic wiring,blue for brakes,and if you have surge brakes with the lock out that is activated by the reverse lights,purple wire there.
you will also need a corresponding number of small alligator clips,an inline fuse holder with at least a five amp fuse.
wire up the new female connector just as it would be if it were on the vehicle.
on the opposite end of the wires,crimp on/solder and tape or heat shrink the alligator clips.
on the fuse holder,alligator clip on one end and male bullet on the other end.
for power you can use a 12 volt lawn mower battery,atv battery,your ice fish finders battery ( it needs used and could probably stand a fresh charge this time of year anyway ).
you can use your vehicles battery if you make the wires long enough but the purpose of this is to get away from the vehicle so you dont have to work around it and a vehicle might not always be around when you want to work on the trailer.
besides,you can impress your friends with the handy dandy tool you just made,be careful though,you will end up testing everyones trailer !!
hook up the fuse holder to the positive post of the battery,white wire from the female connector goes to ground.
plug your trailer and the powered connector you just built together.
now,one at a time,hook each remaining wire to the bullet connector on the fuse holder to test that wires light circuit.
if you blow the fuse in the holder,that circuit is shorted and needs to be found out where and corrected,( this also saves blowing a fuse in the vehicle and not having a spare )
you can now test each circuit individually.
a few tips:
never poke through a wires insulation to test it,this sets up the perfect place for corrosion to set in and now you have another problem waiting to show itself,guaranteed,test at the wires ends only.
if you have the ability and tools,by all means solder and heat shrink any and all butt connections (splice) in the wires length if you dont replace the full length of wire.
if you dont have a solder gun or dont know how to do it,buy the connectors with heat shrink already on them,use a lighter or small torch to shrink the insulation.
do not use acid core solder !!! rosin core only.
solder all ends that go into any connector that utilizes the socket and set screws in them,coat with dielectric grease before inserting the wire and tightening the screws.( talking about trailer and vehicle connector here )
never ever ever use scotch lock connectors,they cut the wires they bite into limiting the wires load capacity and like piercing a wire,corrosion is on its way soon. Grouses favorite connector 
as far as where and how to properly ground the trailer,if the lights you are using have the ground wire with an eye on it already,clean an area on the trailer until you have shiny metal,bolt the ground wire to it and paint over the area.
if your wiring harness has individual grounds route them as required.
at the connector end of the trailers harness,again,clean an area until its shiny,bolt the white wire to it and paint over it to protect it.
while you are at it at the connectors end,make sure you have enough wire so that it isnt strained during a sharp turn but short enough to not drag on the ground.
and dont beat it to death every time you throw the safety chains over it !!
protect the wire anywhere it goes over/through anywhere there is a potential for abrasion or cutting.
never pull a wire any tighter than absolutely necessary,while you are at it,use zip ties at the least (insulated clamps are preferred ) to keep the harness from flapping around or the potential to get caught on something and ripped apart.
use dielectric grease on the connectors pins.
if you are the type that just has to have a sh!t load of lights on your trailer,make sure the wire is heavy enough to carry the load and that your vehicle is properly fused for that load.
so there you go,sorry I ran long and if I have forgotten anything guys,please feel free to add it.
