Multiplex bypass

Which model Volvo?

 

I have a solution for you but it requires a little bit of fabrication on your part but I think you'll like it

 

On the rear of your cab on both sides you will notice a round tubular bracket that bolts to the back of the cab and attaches to the outside fairings using 2 studs which are screwed into the fiberglass fairing (I epoxied my studs into the fiberglass for more strength)
and two nuts which hold the assembly together. If you remove those two nuts and spread the fairings apart you will be able to sandwich a flat piece of metal in between the upper and lower part of the fairing and that piece of metal needs to be bent at a 90-degree angle and have a couple holes drilled into the vertical part to match the factory holes on the reinforcement bracket. Just for the sake of being redundant a short little ground strap about 4 inches long from the metal plate to go underneath the nut which holds the assembly together will provide an adequate RF ground for the metal plate.
And where the plate Protrudes from the fairing on the outside of the truck in between the upper and lower part of the fairing you will need to drill a single hole in the horizontal part of the metal plate to put your antenna stud. The metal plate only needs to protrude about maybe 5 inches outside of the fairing to prevent your antenna from touching the paint on the outside of the fairing. On my volvo 770 trucks I have always used 2 antenna elements mounted on two of those plates one plate on each fairing. To create a directional transmit and receive front to back.
I then ran the cable behind pieces of plastic trim inside the truck behind the refrigerator on the driver side directly underneath the bunk and out through the bottom through a rubber grommet underneath the bunk and up the inside of the fairing on the rear of the truck to the elements. The longer the shaft and the higher to load the further you will transmit.

 

The object is to bypass all aluminum components and the mirror bracket on the Volvo is made out of aluminum which creates ground problems and also putting the mass of the truck in front of the antenna system helps your signal transmit further forward

 

Sorry if I sidestepped your original question but doing it my way is a five-minute installation as opposed to ripping the whole truck apart to bypass the multiplexer

 

It just so happens that you drive my favorite truck and with my antenna setup on that truck I could easily talk 65 miles. Unfortunately I have never been able to achieve the same thing on any other model of tractor but it is easily repeatable on the full-size Volvo 770 780 and 880