Vehicle wiring diagram

Here is a wire diagram of Dodge Ram electrical system it seems your wiring may be either loose or attached incorrectly, check for improper connections and follow the wiring charts it's the best way to find the faulty connection.

https://w05.dealerconnect.chrysler.com/service/mds2002/serviceInfo/en_US/br808154.gif

The obvious problem is that whoever wired the trailer wiring may have made a mistake. A common one is to confuse a ground with a power lead. this will instantly fry the fuse and leave you without power to the circuit. Double check the wiring to see that it was done correctly. It may be a bad ground, however this would not normally affect the dash as it is independently grounded. Find the lead that provides 12v+ to the harness and then check to a known good ground. If you have the voltage, then change the VOM setting to ohms or continuity and meter between your black wire and the known good ground. Anything more than a couple of ohms and you need to re-establish the ground wire. There are many ground tabs throughout the harness...find them and remove the tag bolt (usually 10mm) and clean and re-affix. This will take care of most problems.
Push comes to shove, remove what they did and start over.