minimax

It should be but allowing for voltage drop through the ignition switch and the usual spindly  connections  to car radios, a hard working  set can easily pull the supply down half a volt at its terminals. it only takes .1 of an Ohm at 5 Amps to do so. after all. I would expect to see above 14 Volts on mine as I use good cable and connectors. (Not the hyped up trash for the gullible, thirteen Amp mains cable is entirely suitable for a hundred Watt load.)
I expect that the unit has been turned up to max and wrecked the factory speakers, if I'm honest..