I'm going to say stay out of the car... just because in the garage in my mind your not going to have room to ram or other wise simply escape. The last thing I want to do is trap myself right where a car jacker wants you... in the car.
I'd have my lady friend take cover behind a nearby engine block, if she is armed she better be helping if not, she better be dialing. People also need to be coached on whats important information and what is not in the call for help. Thats a whole 'nother thread.
Then, I'd move off to the side and find a block of my own and get ready to get down to business. I'd try to stay close enough to my companion to defend her position too, but not so close that I'd be drawing fire to her.
There is also no shame in running... this is a bad situation and its bound to get ugly if the fight happens. I'm young and in shape enough to floor hop my way down a few levels to safety, or to be able to reset the fight more to my liking. I would not simply run down or up the ramps, your just going to end up fighting tired and winded.