Manchester (UK ofc), the place where the first Rolls Royce was produced and founded. Could be "tied" to Liverpool (as a single city Liverpool-Manchester). Manchester forms the backbone of the (former) industrious heartland of Britain, the birthplace of the modern society through the wave industrialization back in the 1750's. Manchester being a rainy pit with loads of factories and Liverpool being a... young city turned into the second most important port of the empire, and the most bombed city in the UK after London. I think the ("new") Range Rover Evoque is produced if not in Liverpool, then at least close (I think it's somewhere on the Wirral, which is like a suburb). Ford also placed it's first non-US plant in Manchester... You can also tie Blackpool to Manchester/Liverpool, which is famous for its TVR (Top Gear knowledge).
So there is some automotive history, and probably more if I bothered to discover it.
Population for Liverpool is "Liverpool was granted city status in 1880, and the following year its university was established. By 1901, the city's population had grown to over 700,000, and its boundaries had expanded to include Kirkdale, Everton, Walton, West Derby, Toxteth and Garston" (
http://goo.gl/41NDmf) and Manchester is 1,435,000 (
http://goo.gl/hBYf10). However I think both metropolitan areas have a great deal more citizens, both then and now. However, it should be noted that both Liverpool and Manchester as cities (not that metropolitan areas) have decreased in population numbers as a result of economic decline, problems like riots (allegedly Thatcher wanted to "pave over/demolish Liverpool" as a result of it burning for most of the 80s (and being controlled by commies)), people moving out to suburbs instead of cramped terraces.
Distance is the problem here, the UK isn't big. I guess Manchester is about 130 km north of Birmingham, and Liverpool is not much more than half an hour's drive from Manchester (so 25-35 km?). That's also why they should/could be joint up as a "region". If you have to choose, Lpool is the bigger city, Manchester is the bigger metropolitan area. Lpool is more "interesting", Lpool is also more "important" (it's like water or air, not blue or green, and the symbiosis is enormous) since it isn't as replaceable, however Manchester might have played a bigger role in the automotive history and if you don't replace it, it probably ends up being more "important".
tl;dr - Anyway, both Manchester and Liverpool play a great part of the industrialisation of Britain, starting the fire which first spread to the Belg, France, US, etc and which is still spreading in such places as China.