My cousin, before he retired, was at the top of a UK car manfacturing company and he lobbied hard for Remain, because the car manufacturing business runs on Just In Time (JIT) principles: a lot of parts for the cars come from other countries, many of them in the EU, and because they are ordered to arrive at the assembly plant within a very short time before they are needed, the car company saves a lot of money; for example not having to pay for storage. If the lorries carrying the parts are held up in Customs, this can really mess up production - widget #1 isn't available on time so you can't install widget #2 which depends on #1 being installed first, and so on. The lorry carrying widget #1 might be held in a queue behind a lorry of fish which hasn’t got its correct paperwork. Workers have to wait for the parts to arrive and are, presumably, being paid while they are waiting. I believe that we also send smaller car parts to specialist assemblers in other EU countries, who then send them back to the UK to go into the car. So there could be delays in both directions. It's my understanding that the modern British car industry is more or less based on JIT. (And by the way, in relation to fishing which we get very excited abotu, it turns out that the fish that we catch in ‘our’ waters is not the fish that we want to eat - it goes straight to the EU; while the cod and haddock we want to eat comes from outside out waters!)
And then, on top of all this, there's the separate issue of tariffs on all these parts going in both directions between the UK and the EU. Pity the poor Irish: apparently the ingredients for Baileys cross the Irish border about eight times before a bottle of Baileys is made. Tony Connelly of RTE TV in Ireland has written an excellent and very readable book about this and the other very real effects of Brexit on the Irish on both sides of the border in his book "Brexit and Ireland" - you can get it at Amazon.