How hard it is to place a volt meter where the OEM amp meter is/was varies with year model/ inst' panel form. Hanging a volt mater under the dash isn't so difficult. I saw one volt meter that plugs into a cig' lighter well. Anything is better than nothing.
I have the voltmeter that plugs into the lighter. Works very well. Just pull it and pop it in the glove box at the show. Besides, I always need a charger for my phone. That's a must now days.