This 04 2500 has the trailer tow group. I'm usually using the 7 pin, which was in great shape. But while I was under the back RnR'ing the rear axle, I noticed the flat 4 pin connector looked pretty bad. Well, you can see from the picture it was REAL bad. On occasion I need to grab someone else's trailer and often they are flat 4 wire connections.

Luckily I was able to salvage the truck side of the connector and only lost the cap. This Hopkins cap was one I hadn't seen before, it had LED lights in it that show the connector is good to go. So I thought, I want another one... after a bit of searching I found pepboys had it for under $5 so I grabbed one.

I got the old connector cleaned out and greased up.... seems to drive the LEDs fine, and that is good because I really didn't want to have to splice a replacement in there.

Pics for your amusement...

 

What you replaced was an aftermarket tester. They work okay but you're better off to just put a cap on it. The grease is critical though. When I have a 7 pin connector only, I used a 7 to 4 pin adapter. I makes for a cleaner set up.

 

I tape mine up...Had the same issue when I decided to rent a U haul trailer, and off course on that day, my hitch was rusted to the receiver....

 

You aren't the first to have that issue. I ot a truck once that had that issue. Three days and LOTS of heat with a torch and I finally got it out. I considered replacing the receiver but I'm che...er...thrifty.

 

B F sledge hammer and lots WD40, Coke, Vinegar, what ever U tube would say...
Now I keep it greased...