You want to do this properly? Get psychological about it. You want him to think he's won for now. So you need something that you can do now and will take a seriously long time for him to notice. Some fresh meat in the spare tyre well is a really good one. He'll have no idea where the smell is coming from and then in 6 months when he needs to change a flat tyre he'll have several pounds of rotting meat sitting there.
If you want to cling wrap his car (which is more of an instant thing, not long term at all) then do it properly. Instead of going around it normally go around the middle. Imagine a fat kid lying on his stomach with a belt around his waist. That's the way you want to do it. It means you have to pass the roll under the car but thats easy enough to do. If you're really keen then cling wrap his muffler. Don't do it so he can notice. When it gets hot the plastic will melt and it will NOT come off.
If you have access to his keys you can do some really fucked up stuff. Re-wiring his lights is a great option. He'll have no idea until someone runs up his ass when he's driving. You can also put stuff in the oil spot under the bonnet. Don't muck around with the petrol tank. Its too easy. The oil will really screw the car up. Lots of salt and perhaps sugar will help. Kitchen detergent will work well too. It stays separate from the oil so it'll prevent the oil from touching some of the metal components.
Alternatively get under his car, drop all the oil out of his engine (into a container so he doesn't know) then let him drive around and his engine will seize up.
Another one I saw on Top Gear recently is wiring in an entirely separate radio. You'll need access to his car but essentially unwire the existing radio and put a new one in behind it, connected to the speakers and on full blast with some unbearable music. Wire that up to the ignition and bury it behind the normal radio and superglue it in place. Whenever he starts the car it'll boom and he wont have any way of controlling it.
If you want to do something light hearted then get some stickers that say stuff like "incest is ok" and "I fuck animals" and put them on the back of his car.
If all else fails, put dog shit under his door handles.
