camper shell wiring reading low volts HELP

the 4 pin trailer has my brake light and cap dome lights connected

putting a multimeter in the female side of the 4 pin reads 12v (coming from battery) but once connected to the male reading the volts a foot or 2 down the lines (where i circled) is only 5 volts??

would this cause my dome lights to become dim because they are very dim

so why the sudden drop after the male and female are connected could it be the wires aren’t together good enough. i didn’t solder or tape them yet it’s just copper on copper twisted a bit


up here reads 5 aswell

 

where is the ground connection ?
the bed and shell are plastic so it will need a ground to the truck frame with a dedicated ground wire to the dome light
another cause of low voltage reading could be high resistance in a wire, sometimes the wire can get frayed and corrode inside the insulation somewhere

 

got it dealt with boss yea i grounded it on the frame then put a fuse in the inline fuse holder and it all works now