Hi,
I just replaced the driver side tail light fixture and now for some reason the tail light seems to stay on all the time (when car is on). I bought the light assembly that came with its own harness.
Is there any suggestion about how I can fix this?

Thanks!

The break light stays on... :)

It sound like a cut and dry installation. I would take it back. Sounds defective to me.
You could try and take out all the wires, sockets and bulbs from your old one, and plug it in to see if the same thing happens.

fyi,

Break example= Gimme a break
Brake example 2= when I step on the brake... :)

Just kidding