Instead of keying just piss into a cup and pour it into the vents between the hood and the windshield (around where the wipers are)
It's summertime so perfect time to do it.
He'll be smelling piss whenever he turns his a/c on.
Yeah man put your DNA all over that car.
If that's a concern, just use someone else's piss. Problem solved. You could walk into any public bathroom right now and scoop a bunch of pee out of any urinal.
Also I know niggas that sell piss for drug tests. So I suppose they will sell you piss for this stunt as well.