I did this some time ago on my 2019 S60. Volvo generally has 3 "levels" of interior lighting, in the config of the car. If you have OrBit connected (I know you don't have it currently) you can see Item 183 values for "base", "mid" and "high" as the values that apply.
I took my car from "mid" to "high" which involved very similar process on the S60, install: door pocket lights, puddle lights, rear footwell lights, rear overhead reading lights with the "RGB mood LED" in them. I simply bought the interior door panel harnesses (which is all that was needed) since I was lazy. A note on the harnesses: Volvo built my car without the extra plugs for the "high" level illumination. All replacement parts though, are the version of the harness that has all the plugs, so one part can be used on any of the cars.
I was just as disappointed as you...I discovered that the lights did not work. The config must be changed, or the outputs are not enabled. Then I created OrBit ;-)
Not all were the same case though...rear footwell lights were missing the power feed to the seat harness (but bracket to hold the LED and the wires are in the seat), which is tied to the front footwell lamps inside the CEM anyways, so I tapped the front footwell LEDs for those, no coding needed. The rear overhead light units also work without coding, the added "mood LED" lights worked. Config was needed to be set to "high" illumination in order for the menu to appear and let you change the color though (which worked along with the existing overhead "mood LED" up front, which is already in the car with "mid" level lighting).
May not be exactly the same as the XC40, but general overview of how it's done. You need OrBit to enable the higher illumination level...
I took my car from "mid" to "high" which involved very similar process on the S60, install: door pocket lights, puddle lights, rear footwell lights, rear overhead reading lights with the "RGB mood LED" in them. I simply bought the interior door panel harnesses (which is all that was needed) since I was lazy. A note on the harnesses: Volvo built my car without the extra plugs for the "high" level illumination. All replacement parts though, are the version of the harness that has all the plugs, so one part can be used on any of the cars.
I was just as disappointed as you...I discovered that the lights did not work. The config must be changed, or the outputs are not enabled. Then I created OrBit ;-)
Not all were the same case though...rear footwell lights were missing the power feed to the seat harness (but bracket to hold the LED and the wires are in the seat), which is tied to the front footwell lamps inside the CEM anyways, so I tapped the front footwell LEDs for those, no coding needed. The rear overhead light units also work without coding, the added "mood LED" lights worked. Config was needed to be set to "high" illumination in order for the menu to appear and let you change the color though (which worked along with the existing overhead "mood LED" up front, which is already in the car with "mid" level lighting).
May not be exactly the same as the XC40, but general overview of how it's done. You need OrBit to enable the higher illumination level...