Someone has put the script for the sequel that never got made up for sale. For those of you who didn't knew about this movie. Well, after the success of WFRR the filmmakers starting playing around with the idea for another film about Roger Rabbit. Eventually they came up with the idea for a prequel featuring Roger's adventures as a soldier during WW2 and how he first met Jessica. It was sadly scrapped due to budget problems and the fact that after Schindler's list Speilberg never wanted to have anything to do with movies that featured nazis. (Which if I remember correctly was also one of the reasons why Indy 4 was so delayed)
If you are interested in finding out more about this I suggest you head over to Filmbuff Online which has a informative article about the subject.