I just picked up a head unit for my golf cart, the carts batteries are ran in a 48 volt series, so I would be able to hook up the head unit at 16 volts, Would that be ok.
You'll need to check the installation manual for the head unit. I looked up the Alpine CDE-133BT as a random example and it says 11V - 16V. 16V is quite high for a car so that's marginal.
I had a voltage regulator fail a few years back alt was charging at 19v the battery ballooned up and was toasted but the audio system held up fine with an exceptionally bright display. I have also used a converter to test HU's that 6.0a @ 16v and have never had an issue.