I reckon an early Kia Carnival would make a great platform for a vanlife adventure. It's very unlikely there's a lot of wiring inside the door, and the only electrical wiring that leaps to mind might be a courtesy light for when you open the sliding door, and the little switch that controls it. So, try to find that switch (it's probably down low near the floor) and see if you can see where any wiring enters or leaves the door via the door jamb. Don't expect to see actual wires, as they probably run inside a black rubber boot or sheath to protect them.

Again, it's very unlikely that there's wiring high up in the door near the window (which is where you need to curtain) but if you drill carefully and check the hole for any coloured wiring just as you punch through the metal, you should be fine. A strong torch will help with this, and disconnect the car's battery before drilling just in case you get mighty unlucky.

For starters there is no such thing as a 'lemon law' in Australia; our legislators have chosen to leave us without protection when it comes to cars. You really have no comeback on the dealer, except to appeal to their common decency and help you out. You don't say what went wrong with the car, but the Carnival was prone to head gasket problems in the early days, so I'm guessing that could be the problem.

You need to have it checked on a computer diagnostic checker to identify what is causing the engine to run roughly. It could be one of the sensors on the engine; it could be that the throttle body needs cleaning.