license plate lights dilemma

[hcmor71]

I apparently have an aftermarket rear bumper on my 71 C10 that contains
2 license plate lights. My dilemma is that one side lights and the other side
will only light when the part containing the bulb is grounded against the hole
in the bumper, but when I put the bulb part into the part that plugs into the
bumper, light goes out. Am I missing an insulator or ground ? Thanks for your
thoughts.