....... well lad, it looks like you're in a bit of a predicament now aren't ya?
I figure that you are pretty heavily invested in that crate, so a few more bucks can't hurt.....too much.
So, here's my plan: tell her you have booked her car into the shop to have it's muffler bearings greased and summer air put in the tires before it gets any hotter. Take the car to your work and park it there. Hit up a car rental place and rent her a "courtesy" car. When she heads off to work in the morning call in the rental as being stolen, by the time the cops are done with her and she has proven her innocence your new toy will be in place and she'll be far too rattled to notice the new toy. * IMPORTANT! * If your phone is ringing DO NOT answer it until your ruse has been completed!
Drastic times call for drastic measures....... Good luck!