I think Ashore is right - target the owners, not the dogs.
ApricotTripper has the best idea with the movement activated floodlight if the dog is with it's owner. How about a little "PLEASE STEP AWAY FROM THE CAR..." type recording that plays when the light comes on... something like "PLEASE STOP CRAPPING ON MY FRONT LAWN...". Hmmm, that would be good when the Mother in Law calls after dark too...
My first part time job as a kid was working in the Greek FruitnVeg shop across the road. There was a German Shepard that used to p1ss on the front door every day when it went past. He then waited with a bit of kerosine in a bucket and threw it at the dog when it went to p1ss on the door the next day. It took off up the street trying to wipe the smell off. All the owner had to was splash a few drops of kero at the door every now and then, the dog gave the shop a wide berth. Seems dogs hate the smell of kero. Can't condone this treatment now of course :rolleyes: but it did the trick. (perhaps best not to do it if the owner is attached to the dog with a lead... hmmm, then again....)