Hey I have a 2015 F250 and would like to wire the factory cargo light mounted on cab to work with an upfitter switch.
I have heard of some running a wire under the headliner and teeing into the existing cargo light wire and using a diode to keep from energizing the interior light.
There is surely a better way possibly tying in closer the cabin light switch on the dash or does the wiring not split off until past the interior lighting?

 

Warning - Ford controls most of their lighting with a field effect transistor (FET) for each lighting circuit. If one is not careful, they can overload the FET and destroy its internal workings. The FET's are located in the BCM. The BCM is NOT serviceable and if you damage a circuit, you would need to replace the BCM.

You didn't say if you have a Crew Cab or Super Cab or Regular Cab, but they all are very similar. The "cargo lamps" are controlled by an FET. You can power the cargo lamps with an upfitter switch BUT, I would protect the BCM with a diode. The diode prevent the voltage applied from the upfitter switch from reaching the BCM. The diode would allow the voltage from the BCM to power the lamps during normal operation.

The diode would be inserted on the BCM side of the connection point for your wire form the upfitter switch. You should be able to google and find instructions on how to diode protect a circuit.

The wire color is Grey with Violet (GY-VT) stripe.

Just my humble opinion and 2 cents - about the diode protection.