Having been let down big-time when my alternator failed, I discovered the hard way that there appears to be NO WARNING if the alternator fails completely.
Accordingly, I fitted a permanent voltmeter into the dash - and the ten minute job took me a couple of hours.
To make more space to get a hand in, first remove the glove box — 6 screws; 2-top 2-bottom + 2-inside to left and right. but see below
Also remove the panel above the leg air bag and unclip the two lid restraints.
Be careful removing the plug to the passenger air-bag switch.
Pull/lever out the mirror switch and unplug. Locate the green/yellow wire on pin 7 (earth) and the fawn-coloured wire on pin 2 (+12v when ignition is on). Splice in two wires to supply the new voltmeter — I found the wires rather short as they were in a tight loom so extended them a little (under the heat-shrink).
Drill the hole to fit the voltmeter. I used a large cone (taper) drill to cut the hole in the plastic dash - the meter I used needed a 28mm dia hole.
Fix the voltmeter in the hole, connect up and test. 11.7 volts is low - not a meter fault as this is confirmed by other meters. I suspect something is going on as the battery is new!
Having done the first one, I found that, with patience, it was possible to avoid removing the glove box. After removing the mirror switch, the hole was just big enough to add the two extra wires and to connect and secure the voltmeter.
Drivers eye-view of the resulting voltmeter fitment on the yellow car.