Hard Wire an Accessory

I guess this might have gone in the Montego area, but it might be missed there by someone with the expertise I require, so please forgive my posting it here.

I just purchased a 2007 Montego, and would like to install an electrical accessory that I only want to work when I start the car.

The dealer, says that the car uses a different wiring system that uses something called Ground Switching.

I have tried to ask other people, but all tell me this sounds like bunk, and the dealer just wants me to pay one of their mechanics the hourly rate to attach a hot and ground wire.

Would someone be so kind as to help a "dumb broad" by telling her where to attach a wire (I thought in the fuse box) so it will only work when I start the car? I figure a ground is a ground (right?)

My gateful thanks in advance.

Denyse