Not a good idea splicing into any OEM harness. All are CAN BUS (Controller Area Network) type lights and are controlled by the ECU. Your Hyundai warranty precludes this type of modification to OEM electrical wiring harnesses and you risk having an electrical warranty claim denied.
Hey dont know what u ended up doing but u could get power from battery into a relay and use a usb cable to trigger the relay. That way when car turns on so do interior lightning. Also with some further tinkering with an arduino u could get a light sensor and program it to trigger the relay when sun goes out.
I'll repeat the above sentiments; you can't change the dash lighting as it's part of the dash panel itself. The lighting is part of the BCM and canbus systems, don't mess around with it. Change the bulb overhead bulbs to LED's but leave the rest alone.