The light strip that goes on the door is basically a tiny LED illuminating the optical tube or a acrylic sheet. its soldered to the flexible pcb and has a current limiting resistor. If the resistor or the LED is bad, it would either glow dim, flicker or go completely dead. Loose plug or disconnected plug will not cause a low brightness issue but rather completely go dark.
Thats the reason i suggested replacing it out completely, ideally as a pair (both doors) so the brightness is uniform.
If you are not comfortable diagnosing it yourself (pulling out the door card, prying out the trim, etc) have it done at the shop.