Home Depot trailer wiring... whack?

We tried to rent a HD 5x8 trailer this morning but when we hooked it up to my truck its lights didn't work. I should mention the trailer is a 4 pin, whereas the Taco has a 7 pin. So I had bought a 7 to 4 pin adapter at HD.

I was confident the plug on my truck works. We got back a few weeks ago from 3000 mile trip to pick up our new camper and checked the trailer lights daily. So, we figured the 4-7 adapter was faulty and went in to get a replacement. I also bought a different style adapter, just in case.

Tried again, and neither adapter worked. Confirmed we had it connected correctly (you can only put them in ine way). We then moved over to another trailer and its brake/turn signal lights didn't work either. Flummoxed (and without a multimeter with me), we cancelled the rental and went home.

Once home, I tested everything - I hooked the truck up to the camper - lights worked, used a multimeter on the truck pins - everything checked out, put each of the 2 adapter plugs in the truck connector and tested with the multimeter- both showed proper voltage on each blade.

So I guess HD is renting trailer with faulty wiring with no brake/turn lights? Guy assured me they worked for the last renter... Could it be anything else? If I go back, how can I test the trailer light to see if they are getting voltage at the bulb. Whole morning shot on that fiasco.

 

Alot of trailers don't have the Ground connected but instead ground through the ball, if there's a little rust on it there won't be a ground to the trailer until it cleans off.
I don't know why that is but I've seen it alot on trailers.

 

A few months ago I rented a log splitter from HD. The flat plug was cut off and the tail light wires are also cut.