The mice are very aggressive where I live. Poisoning them doesn't keep them out of the car, it just ensures that they live long enough to get into the car before dying and leaving stinky remains. Also, the poison doesn't act quickly so if you don't keep replenishing it the mice develop a tolerance. I now use the Car Jacket (carbag.com) when I store my cars because nothing else will keep the mice out. It completely encapsulates the car so you don't have to worry about anything getting in.

Just did some reading on that product and I may need to get it once I finally get all of the rust removed from my 240D. Pretty cool.