Re: light Q and what gear??
If your trailer lights are not sealed, you are backing in the water with the brake on so the brake lights are on. When the hit the cold water, poof!!! As for parking on a ramp, if you have an automatic transmission, with your foot on the brake, first set the parking brake, slowly release the normal brake to see if the parking brake is holding. If it is, then put the transmission in Park. If not, you have no choice. Putting an auto tranny in park and letting the load roll back against the parking pawl causes a very loud bank when taking the tranny out of park. In some severe cases, you may not even be able to get it out of park and will need a slight forward tow to release tension. That bang is doing damage to the parking pawl and eventually it will cause slippage rendering the park position useless. I've seen broken transmission cases, parking pawls that resemble meat cleavers (no teeth), broken shift linkages, and have had to rescure folks who can't get the vehicle out of park.