Continuity mistake: When the cop car comes out to chase the Blues Brothers from behind the sign, the car starts off directly behind the sign. When the Good Ole Boys pass in the Winnebago, we see the cop car is before the sign and already has a running start.
Continuity mistake: At Chez Paul, when Jake and Elwood first meet up with Mr. Fabulous, who is talking on the phone, there is a shot of them walking down the hallway towards him and you can see that Mr. Fabulous is looking right at the two brothers. In the next shot, his head is down and he then looks up at Jake and Elwood.
Answer: It's 'Can't Turn You Loose,' often refered to as the Blues Brothers Theme.
rabid anarchist
Also done much earlier by Otis Redding.