I have to replace the 7 prong plug on my larger trailer , half of it is worn away from road rash on a New York trip . Tractor Supply sells these plugs . My question , how to find out which wires go where as I only have half a plug and the plug is sealed ?
The first answer is a simple ohm meter. By the fact that you're asking, while I'm quite sure you know how to use one, I'm guessing that electrical calculations like counting filiments and estimating electrical loads in your head is probably not on your list of things you enjoy.... So here's the (slightly) longer method. It uses a battery, masking tape, and a couple of scraps of wire. You could put a fuse in line if you wish, but you really can't "kill" anything (including yourself) if you make your first couple of "contacts" quickly with a brushing motion. You'll know right off if you get a "lighting some lights" spark, or a dead short "you're doing it wrong" spark. This will "break down" the trailer circuits into individual pieces so that you may eliminate 5038 of those 5040 possible combinations. I say it's "longer", but it probably will take you as long to read this as it will to actually do this.....
Never rely on a color code. While all the plugs are wired the same (usually, go with that standard.....), there is NO official standard for this. That means manufacturers (and any previous DIY or even professional repairs) have no color standard to go by. There are some "more common" colors for functions, but there are several common systems, and NO guarantee that any standard is followed. Use the actual function. It'll save your sanity.
First, find the GROUND wire on the trailer. Follow that one to the damaged plug. Attach that to your battery negative.
Second, Functionally verify the ground. Find another wire and quick touch, then attach it to the battery positive. Go and see what lights up. Don't do anything with that yet. If nothing lights up, it might be your brake wire. Ignore a null result FOR NOW. Try a different one until you get lights. Even just one light. Leave it hooked up. Now, find a SECOND wire (again, skip over any null results), and while still lighting the first function, light another function.
Option one- You get two discrete lighting functions that work as they should. You can carry on. Option two, you get lighting results, but they are not discrete functions, you get odd combinations of things. That will indicate that you've misidentified the ground wire.
Third, remove the two wires from the battery positive. Use the tape to make a flag on the battery negative wire, and mark it as such. Attach (one at a time now), one additional wire, holding out for a lighting function, ignoring null results. Observe the function, make a flag, and label the wire. You now have two wires identified. 5040 possibilities just got reduced to 120 possibilities remaining. Now repeat until you find the remaining two lighting wires. (They are ground, tail, left turn, right turn. There is no "brake light" wire.) Once you've identified those four wires, 5040 has become 6 possibilities.
After this step, you will have four wires identified. Ground, tail lights, left turn, and right turn. (The turn signals will burn steady, they won't blink). Also note, the turn signal and the brake lights are the same two wires. In other words, the brake lights are the left and right turn signal burning steady at the same time. So that function sorts it's self out.
When those are sorted, the remaining wires.... Could be only one depending on the age of the trailer, probably two, and possibly three. Those functions, brakes, auxillary power (to charge a breakaway battery), and reverse lights. Being a commercially available harness instead of a hand made one (eveidenced by the molded plug), you MIGHT have extra wires that are unused. That'll mess up your day..... But you can't hurt anything... Power them one at a time with your battery, but put the battery on the trailer. Attempt to move the trailer. One wire will lock the brakes. That is your brake wire. By the nature of electric brakes, it takes some degree of wheel travel to activate the brakes, so go a couple of feet each way before you give up, so you might want to attach it to a tow vehicle for this. Just don't plug in the partial harness yet.
So now, worst case, you've got two wires left. Two possibilities. One to charge the battery, and one for the reverse lights. You could brute force guess at this point, or if you're missing wires (less than seven), process of elimination might tell you what feature is left based on what you have.
Reverse (which you've found if you have that feature, but probabbly not), and battery charge/auxiliary power. Now you need the meter. Or a light bulb. Or the sketchy way, raking a wire lightly and quickly across a connection to see if it acts "shorted". One of those two wires goes straight to the positive terminal of the breakaway battery. The remaining one (assuming you probably don't have the feature) is a dead end wire that "could" power reverse lights.
As with most things in DC electrical systems, the key to make it work is to break it down into pieces. Working in a systematic way will get you to where you need to be.
How bad could it be
Usually I have a 50/50 shot at wiring and see magic smoke appear before my eyes . 
