No idea, and you could well wreck it by trying to inject a DC voltage (not to mention invalidate the warranty). You’d need an inverter. 

The DC voltage in an auto / marine system is not well regulated, at a minimum to keep from burning things out you’d need to add a voltage regulator that could both boost and cut the DC input voltage to keep the output voltage stable. Dealing with the voltage spikes while cranking the engine will be fun too, get that wrong and you’ll likely toast things.

Seriously, go with the inverter and a decent quality one.