Big truck dual gauge cluster trivia; RED plug and delete plate
 

In checking by Big Truck dual gauge cluster I noticed that the 2nd plug on the back is a GMC Red looking color and not grey. No way to mix those two up when connecting.
Also I found a photo of a cab over truck that did not have a second cluster but instead a delete plate like you find for radios. I'd not seen that before.

Re: Big truck dual gauge cluster trivia; RED plug and delete plate
 

Here's the delete plate