The Volvo has a voltage display on the dash. However, when a message is present like "Low Fuel" or "Low Air" the voltage display is covered. I also noticed the voltage display is a little lower than the actual voltage. I'm going to install an analog voltage gauge.
I had previously removed the remnants of a Qualcomm installation and found my 'accessory' power circuit had been modified. The accessory wire had been cut near the ignition switch to power the Qualcomm. The circuits that are normally accessory powered (radio, windows, mirrors, etc.) appeared to have been factory jumpered to be powered constantly.
Since my 'accessory' power wire was easily accessible and hooked to nothing else I decided to use it as the source for my voltage gauge. I connected it to a terminal strip I installed in the fuse area. I also used the dash light circuit from an unused switch plug to power the voltage gauge backlight.
The hardest part of this project was getting these two wires into the overhead console. Here are the wires going up the A-pillar.
I thought it would be simple to snake the wire into the dead space in the overhead console. I was so wrong. When the wires leave the A-pillar they make two bends to get to the dead space. I tried for hours to snake the wires through. The bends were too tight and narrow. I eventually drilled a hole and ran the wires through the back of the overhead storage compartment.
I will install the gauge where a power point used to be. Drilling the hole for the gauge was a trivial part of the project after spending hours snaking the wires.
I used a VDO gauge. I installed a Carling switch to switch power to the CB, dashcam, radar detector, etc., that I plan to install. I used the powerpoint lead for power and ground. If I need more power there is also a CB power lead available.
While I was at it I installed a Cobra WX75 CB.
Although it was a bigger project than expected I like having the voltage gauge. I can flip the key to accessory and check the battery voltage without turning anything else on. When I start the truck I immediately know the voltage without having to wait on the dash display. It also more accurately displays the battery voltage.
Update: The WX75 has a lot of nice features but one thing it doesn't have is much volume. I replaced it with a Cobra 29.