If I have a crappy credit card can I still rent a car?

yes but a deposit must be left it will be charged to the card

It depends on how you define "bad" credit card. If it's a standard card (like Visa, AmEx, etc.) it simply has to have enough room on it to hold the reservation and possibly a deposit. If you have a "bad" rate on it, that won't matter unless you don't pay your bill on time. Being 25 or older is a bigger concern to the rental companies.

If you need a cheaper rate, I tend to do well with Hotwire in these cases, since most people tend to rent from O'Hare or Midway.

Credit has nothing to do with renting a car.

try budget, thats who i use while in chicago, they take debit cards as long as a blance of i think 300. for deposit, until you return.