That will turn the light on and off when the doors open and close. And if you leave the switch in the circuit you can use it to force the light off, even with the door open.
But if you want to reach up and turn the light on with the doors closed, the switch isn't going to do anything.
That detail was important to me because I wanted a second dome light in the back of my Suburban that I could control independently. Now I can turn on the forward or rear light from the stock switch, or they all come on with the doors open.
However, if you wanted to duplicate the feature many modern cars have, where the dome light stays on a while before going off, either when you insert the key, or after a minute or two, then the relay approach makes it easier to add a timing circuit.
The Mad Electrical stuff looks helpful with the instructions they offer.
All Electronics had similar relays for $2.00 each last time I ordered some, though they don't come with the detailed instructions. The same people have some cheap lock actuators too.
