Third Brake Light partially working

Hello,

I have a 2008 Cadillac CTS and my third brake light (on top of trunk lid) isn't
working as it is supposed to. When I press the brake pedal, it illuminates, so
it is working properly in that sense. But when I unlock the car using the
remote, the 2 taillights flash but the third light isn't flashing at all. I am pretty
sure it used to flash.
Note that I recently fixed it from not working at all, after a full car inspection
from the mechanic and he told me that it wasn't working. I didn't check for
myself to confirm, I should have done it. I checked the harness where the
wires bend between the trunk lid and the top of the trunk and one of the
wires (Black) was cut in half. I just spliced it, checked, and it was working
fine. Now it is only the flashing when unlocking the car that I can't figure out.
Any idea what the issue might be?
Any help would be greatly appreciated. Thank you.