Who's "they"?
My UC's were both the same, as was my friend's '09 SERG.
There may be a different overlay for the SEUC because of the radio amp harness, but, I would take a look at each.
My adapter was plug and play, except for a ground wire and a hot (fused) lead.

"They" are American Legend and a Co. called Electrical Connection who sells the wire harness to Bushtec.
I did pull my seat off today and my harness off my 05 plugged right in. I got the power tester out, and I have running, brake, and turn signal lights.
I called American Legend back to ask why this harness wouldn't work. I got about 3 different answers.
Now, my trailer is out being painted, so I don't know if all the wires are the same going to the trailer. At the very most, I might have to switch a couple wires around on the trailer plug.