Interesting thought, johnofwhixall.
We don't even know what deal the car makers have been offered, apart from the fact that it must involve a great deal of that £350 million per week.
Even if we do a deal I cannot see it particularly favouring the car manufacturers, especially as a great many of their components come from other EU countries, sometimes crossing borders more than once as they are manufactured, so unless we stay in the Customs Union this is going to bite straight away (or are the sweeteners big enough to compensate for customs duties and delivery delays?). Someone (was it May?) spoke happily of the UK developing its own components industry so as to circumvent the tariff and non tariff barriers, but I really cannot see that happening in the space of two years. R & D alone must take far longer, let alone building new plant and training new workers.
I wonder, if most (or is it all?) of our motor manufacturers are foreign owned, would the views of their EU owners (i.e the French) have any influence on post A50 negotiations?