I sure hope it is!It likely is wired in parallel with the other light.
I think you meant in series. But, both the middle and the inner would be dim as they would both only be seeing half the battery voltage.
How dare you ask this here! Afterall, we have a sister Impala Tech site! l
Couple of thoughts....You may have the socket grounded good, but how good is the ground to the sheet metal? If memory serves me correct The inner and middle are apart of the trunk lid. Did GM run a dedicated ground to ensure the lights are grounded, or did they just depend on the hinges and latch to make "enough" ground. You're pulling more current now.
Also, if this is an original car, maybe the inner wire feed has a questionable connection so the extra current being pulled thru it is now showing up as a dimmer lamp. Bulbs are like snowflakes, they maybe the same type, but some can appear dimmer then others when there is a voltage feed issue.
How is the tail light brightness??? I'm assuming you're seeing the brake/turn brightness being affected??
Anything different between the left and the right trunk lid mounted lamps/wiring/factory grounds????