WHY IS IT THEY DO THIS? This actually happens quite a lot, in all areas where the grey squirrel has populated. Most of the time, the reason is overpopulation - there are just so many squirrels out there that these places become attractive.  Sometimes these are young and inexperienced squirrels or those that have been RAISED in a place such as this.  We have found squirrels nesting in every conceivable place, from large industrial fans and equipment, to residential vents, fireplaces, grills and vehicles.  Especially abandoned or little used vehicles, although there are squirrels that do this in vehicles that are used every day.  Although I'm sure others might, I have not come up with this except with the grey squirrel. OKAY, SO WHAT DO I DO? For the present, you can open the hood up and leave it open so they will have no cover.  (They don't like this.)  Or, you can always try parking down the street, well out of the animal's comfortable territory.  Even though this might put you out, somewhat, it might be easier than paying several hundred dollars for new wiring.  Which, BTW, are charges that are generally not covered by your insurance.... DETER THEM? Well not really.  Short of staking out a large dog within range of your car.  Let me put it this way: there is no device, chemical, mechanical, magnetic, electronic, sonic, stroboscopic, or any combination of those, that will consistently keep these animals at bay. Neither will loud rock music, stuffed owls, marigolds, mint, basil, catnip, boric acid, bleach, ammonia, soap bars, Bounce dryer sheets, urine of wolves, coyotes, fox, bear, or dinosaur.  Nor will dog-do or even human-do.  Did I forget anything? You have to do it the old-fashioned way.  And that way is to trap and remove them, and that should be your ultimate goal.  And I'll always say that this is also the reason not to encourage (by feeding) these, or any animal - ever.  Any wild animal, habituated to man, becomes dependent on him.