I Have replaced both of our 2012 3500's with Salvage yard Gauge clusters, We have reached 636,000 miles on each truck and the odometers quit working, 1,000,000 million Kilometers.
The answer you are looking for is, at least on a 6.7 3500, the wiring behind your gauge cluster should be there.
Disconnect the Battery's, You can easily scratch the cluster, Take your time unplugging your cluster, You may have to adjust your steering column up and down to work out the old unit, You May have to "tug" on the Harness a bit, while its still plugged in to the old unit, to get enough working space, to me the "Brown" plug disconnects the Hardest.
I have purchased my "Used" Gauge Clusters from the "Coparts" venders, most of the clusters Have the Mileage stated, We have since upgraded to the "Laramie Longhorn" Clusters for both of our 2012 Ram's, "Just a nice looking" Gauge Cluster.
I have put into several friends trucks, or by word of mouth that I know how to Install these units, other gauge clusters from salvage yards, I will show these folks the steps and let the fella's do the work.
We live close to an area where "RV Trailers & utility trailers are manufactured, Fleets of 6.7's "RAM's", many with over 1,000,000 miles in our area.
Good luck on your "Swap", We are Simple Farmers in Mid-Michigan, If "I" can do it, you can do it, and be proud of your accomplishment.
Jim Droscha