The interior lights are fed from the central control unit module which takes inputs from door switches, ign switch etc.
If you want to override the lights then you will need to connect to the lamp itself which already have a switch to earth when you switch to ON manually. This would then remove the ccu control. If you only want to switch them off when they are on then that is easier, purple is 12v and purple white is earth. Break one of these lines and they will be off until you switch back on then they will be ccu controlled again.
Make sure whatever mod you do you don't put a feed on the ccu output otherwise you'll blow up the module.