Threat assessment made and decision made to retreat(your words)...I would quickly scan the rest of the parking lot with special attention paid to the retreat route and all the hard cover points.
First hard cover would be the nearest one possible and if possible, somewhere where you could keep an eye out on both individuals (the one in the Camaro and the one behind your car.)
To get the jump on them, I would make like I forgot something and say loudly, that you left something in the restaurant and how the both of you need to return there.
Hand stays on gun to return immediate fire or to lay down covering fire , until you reach the restaurant to call in the cavalry.
Just as an aside, some 25+ years ago, I asked my wife to watch my back when I was loading in some groceries in the back seat. (We were in a questionable neighborhood)
Imagine my surprise when I stood up to see her dutifully standing on the sidewalk intently staring at my back(not looking right or left at any of the passer's by. When I asked her what she was doing, she said, "I'm watching your back just like you asked."
She now carries a S & Klinton 3913 everyday and does well with it.
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