At a stuck point

Hi Guys
I am running new wires to the trunk area for all the lights.
I was not able to find sockets with three wires ( one for ground) but a member here suggested I solder a wire to the socket for ground.
Please look at the picture this weather tight connector goes in a hole in the trunk, and it appears the wires are molded into it.
I would like to keep the water tight connection, where can I find these, or is these another idea.
I am afraid just a grommet will not be water tight

thank you
George