How do you tell if it's the trailer or the pigtail?

There's an easy fix for that, assuming the pins are the split type and they are intact, without missing one of the prongs. I've been doing this for years, and it works every time, unless the pigtail is actually worn out, or there's a broken wire, etc.
Sometimes it's just a matter of a good cleaning instead of automatically replacing the pigtail.
If you don't have one, pick up a cheap pocket size flat blade screwdriver. I have one and leave it in my door pocket.
Then get a can of non-chlorinated brake cleaner (any auto parts store will have it).
If the lights are flickering, spray both the pigtail and the trailer 7-way clean with the brake cleaner (use the attached spray hose). Then very gently so as not to break them, use the screwdriver to spread the pins ever so slightly. This will give you better contact for awhile, at least until it can be determined if your 7-way needs replacing (possibly worn out).
Some people I know also like to carry some dielectric grease to put in their 7-way.
I can do this kind of cleaning in about 1-2 minutes, usually it's all I need to get it to work.

 

The trailer is a rental or I would do it.

 

I meant if it’s the pigtail. If you’re having to zip tie to multiple trailers, it’s your pigtail. Put a new plug on it. They are like $8 at the local parts house.

 

That's not a good solution. I used a rag once to force a container to light. Tunnel police pulled me over east of the tubes in Baltimore once. Saw that rag and let me go. But it had to be fixed.

A side thought on the newer connectors, why in the world must they move all the circuts around over and over and over? Just stick with the old one from way back when sheesh. So everyone can remember the patterns.

 

I believe moving wires around isn't planned. It is the employees who don't care and put any wire in any socket, tightens the set screw and moves to the next plug.

 

Sometimes the set screws inside the receptacles loosen and fall out over time and the wires are now floating in the plastic. When plugged in they don't make good contact because one wire may push the other back instead of connecting.
I had this happen, easy fix and costs nothing. I took apart all the receptacles and re-tightened all the set screws. takes about 15 minutes.
Worth it to check that anyways.

 

Ya,agree with Nightwind,just back up close to another trailer and see what happens.A lot of times (its the corroded/dirty pins on the board.They get bent,spread out also.Im not above(jamming a piece of cardboard off a logbook in between the pigtail and receptacle.

 

On the trailer side. There's not much that can be done with the pigtail other than clean them: if the pigtail is worn out, it's worn out. Better to just go ahead and replace it. Carry a spare 7 way with you, or if your company can spare one, just carry a complete cord with you, either a brand new one, or one that has had both ends replaced with new plugs.