I am not sure if size is an issue, if it is not there are encapsulated relays out there called PAM-2 and PAM-4. Basically these relays are encapsulated in epoxy with pig tail leads. You could submerge them in water without damaging them and they are pretty cheap. The other relays i have seen being used for headlights have heat shrink, they look smaller but these are better protected. They run around $8-$9.
The PAM-2 coil can be controlled by 12 or 24 volts DC. It also has a red LED that comes on when relay is energized.
The PAM-4 coil can be controlled from a voltage range of anywhere from 9-40 volts DC. It handles more power.
They are the same size: Dimensions:1.5" H x 1" W x 0.88" D (3.81 x 2.54 x 2.2 cm)
They only consume about 15 mA of current which isn't much at all.
I have used these many times in different applications, they last for many years even when they are on 24 hours a day.
You can find them under several brand names: Space Age Electronics or Air Products and Controls are just a couple.
The load of the headlights wouldn't be much at all for these relays.
Just another option.