What size solar panel, what wattage, what voltage, what amps? All these play a part in what goes into your 12v plug. I have a 100 watt factory installed solar panel on my 2017 Trend, which uses a charge controller to control amount of amps and voltage into my coach batteries (AGM). IMHO, you would over charge your engine battery and ruin it, if you did not control input. The voltage/amps may heat up the cord or blow your fuse, or overheat your wiring.
There is more information out there for solar panels and what is needed for charging batteries.