[Drew]

On your original question, as Bestatches said, all you need is a 7 pin to 4 pin adaptor. The factory hitch comes with a 7 pin, and it sounds like your trailer is a 4 pin type. You can buy the factory adaptor or from a place like Walmart.

As for the relays, the factory hitch comes with them, so I'm pretty sure your dealer installed them when your hitch was installed. Easy to check though: look in your fuse box, next to the battery. The lid of the fuse box has labels to show the spots for the 2 trailer fuses. I believe they're labelled "Trailer 1" and "Trailer 2."

On the 'wiring harness,' are you referring to the harness for the brake controller? If so, don't bother buying one because Titan XE's do NOT have the brake controller wiring preinstalled. Nissan put out a TSB that states this. You'll have to have the brake controller custom wired in, which is what I had to do with my Titan XE.