I finally got round to fitting a digital volt meter to my 300Tdi at the weekend. The meter came from eBay (item no 350809740588 looks similar). Easing off the panel containing the electric wing mirror control, I removed the faceplate and some of the material to the left of the joystick to mount the meter. I found a spare connector in the loom behind the panel and got an earth connection there, the live feed and sense feed to the meter were both connected to the live feed to the mirror control. Meter shows just over 12v with the ignition turned on and rises to about 14v when the engine is running.