Title is a quote from Orwell's Down and Out in Paris and London. In this section he goes through a bunch of London slang terms and what they mean, but I don't understand his definition. What does it mean to watch vacant motor-cars? The only thing I can think of is someone who you pay when you park who makes sure nobody steals your car when it's gone. But how does that work? What assurance does the driver have that the person watching won't just leave or steal the car themselves?
Orwell attempts the etymology of his slang, saying of glimmer
'Glimmer' (with the verb 'to glim') may have something to do with the old word 'glim', meaning a light, or another old word 'glim', meaning a glimpse;
Down and Out was published in 1933, a memoir of his time spent destitute in London in 1927 and Paris in 1928. He seems to have missed the slang word glimmers (also glims) meaning eyes, words current in London slang during the 1920s according to Partridge's A Dictionary of Slang and Unconventional English.
So a glimmer is someone who used his glimmers to (in modern parlance) keep his eye on something, in this case your car when you leave it somewhere dicey. Presumably this works in the usual way: you pay half up front and the other half when you retrieve your car undamaged. You have no assurance that the glimmer won't be satisfied with half or that he's not working for a chop shop.
glimming n. [glim v. (2); note London cab-driver jargon, glim, to look for a cab] (UK Und.)
watching out for cabs etc. for wealthy people, in return for a tip. ...