Well, depending on what sort of car it is and how you plan to install hal into the car it wouldn't be too difficult to control the lights, wipers, or trunk. The way I would do it is by sticking a small computer somewhere in the car powered by the car battery, and then anything electronic in the car that hal should control would be always on, as far as the switches in the car, and then a usb or COM power switch controls whether or not the power actually gets to the part of the car to be controlled. I had an idea for putting a media server into my car a while back which worked on similar ideas, but I never finished it as I ran out of time and never got back to it. All of it depends on where your expertise lies though, as I would think it would be easier to control the power in the car, but it could be quite elegant to actually control the wiper motors and pre-existing switches to the lights directly with a servo.