Side mount - or roof mount? Permanent - or something you open and close? Water proof when opened and connected, or when closed? Obviously different environments and considerations.
You'll find a lot of "water resistant" claimed waterproof, adapted cigarette lighter style plugs with a rubber/poly cover. These hold up fairly well in a marine environment, for several years. But you'd be opening and closing frequently, these will wear faster.
Anderson power plugs are great, but you'd need to place in a box or container, or rely on the rubber cover:
Another polarized connector is available, but would pool water in a rooftop mount:
Recently I picked up an outdoor extension cord "cover." I'll use it to connect my external radio antenna. Could be a solution, but it is just a cover and you could use "standard" waterproof solar panel connectors inside it. But they might be a bit large for your needs:
Another option might be an external 12 v post inside a watertight box:
But these are not "plugs" per se and may not be answering your quest.
A 2-pin "Cannon Plug" could work for a side mount. These appear to be "keyed," but unclear if the key maintains polarity:
This last item is one I've considered for similar use could be worthwhile:
Unfortunately, it doesn't show what the type of connector is inside.
Best of luck.
Cheers - Jim