Big Mak - I installed mine the same as yours.
Under the drivers seat there is a slit already in the carpet, and it fishes easily to the rear seat pan under the carpet. But instead of going over the edge of the seat pan, there are several large holes that the cable will pass through, then out the punch under the black mat.
I used a Cobra 29, sideways mounted as yours. Since its the night watch whatever, it is silver white in the day and green at night. Matches all the rest of the Factory and Autometer gauges nicely.