I've been working on this for so long that I'm kind of sick of looking at it, but it's finally done, thanks to another giant snowstorm covering Philadelphia in 2+ feet of snow and effectively canceling all my other Saturday plans.
The only thing I don't like about working in Illustrator is that the end result can look kind of sterile, so I decided to play around with adding texture to this when I took it into Photoshop at the end - I think I like it. I'll definitely be experimenting with that some more. Overall I'm pretty happy with how it came out, even though I keep going back and forth about whether it's too dark... it's a night scene so most likely not, I'm just so used to everything being light and bright.
Anyway - here are the sketch and thumbnail I was working from. I don't think I've ever actually planned a background out beyond thumbnailing.