I've only done it the other way round - i.e. put in an isolator so that the lights don't come on even when the doors are open - for camper conversion, but:
As the lights come on via a switch controlled by the side and rear doors, you should be able to add an electrical circuit connected to each side of that switch (i.e. in parallel with it), with a normal e.g. rocker switch, to bypass the open part of the circuit (when the door is closed), and thus keep the lights on when the door is closed.
Be aware of the possible issue of invalidating your warranty when you tamper with the electrics.
Or you could add a complete new circuit from the battery under the seat, properly fused of course, with switch and new lighting for the interior.
HT