Misti, I do know a lot about how the car industry works, so allow me to elucidate for sore arse with reference to specifics.
"so we'll buy more Nissans."
50% of all Nissan cars manufactured in the UK are done so from parts imported from the EU. There would be tariffs on these parts as all automotive products are tariffed under WTO terms at 10%.
This dramtically increases the costs for Nissan in the UK, and will be added to with WTO terms because outside of the customs union the non- tarrif barriers also increase costs of production. This will increase the cost of a Nissan to UK consumers and EU consumers at the same time.
In the UK this means that the competitive advantage of the WTO tariff is removed, and in the EU means that Nissan will find that consumers will purchase other cars ( but I'll get to that in a moment).
75% of all Nissan products manufactured in Sunderland are for EU consumption. Nissan in 35% owned by Renault. The ease to switch production of cars for the EU to avoid these increased costs for Nissan is extreme, they have a plant in Spain that could easily increase capacity.
These issues with costs are clear for all other volume producers of cars in the UK. No customs union and tariffs, costs rise significantly to continue producing for the entire EU market here, and even if they leave production for the UK market here they don't obtain much competitive advantage for doing so because of the need for imported parts.
That goes for Vauxhall, Nissan, Toyota and Honda.
Finally the "German car industry" point is utterly facile because.... The big car firms know that demand for Mercedes, Audi, VW and BMW cars is relatively price inelastic.
The evidence is clear, these cars marques already price themselves significantly above the competition in each class of car, yet are market leaders. For example the german made BMW 3 series starts at £26, 000, yet out sells the British made Toyota Avensis which starts at £18,000.
Now you could have the Toyota with all the bells and whistles on it for less than that you could buy the basic diesel BMW. Yet the BMW still outsells the Toyota.
Why? Because price isn't the major determinent of demand for these cars, where as for Nissan, Toyota and Honda it is.
Landrover/Jaguar will be fine, but the others I'm not sure about at all.