Not that serious buddy!!
NEVER USE SO CALLED "PROFESSIONALS" THAT BUTCHER YOUR CAR AND CHARGE BIG $$$'s FOR SHIT WORK.
You will need to remove the head unit and relocate the GROUND wire of the head unit.
They have spliced into a wire that is a "fulse ground", as when you turn the lights on it becomes 12V, this wire should be used to dim your stereo (if it has that feature). Easist fix is to disconnect your stereo ground and secure it to bare metal (chasis point).
There should be a blown fuse, do not know where it is on a Skyline, make sure you check all fuses is all 3 locations (Engine bay, under dash and boot (near battery).