I know of 2 such films from the mid and late 80s:
This is one: http://www.imdb.com/title/tt0094769/combined
This is the other: http://www.imdb.com/title/tt0091909/combined
My guess would be that Blue Movies
is the one you're thinking of. Major Burns, from M*A*S*H (TV) is the guy they hit up for money, the professor from Gilligan's Island is the local mobster, the lead girls are a goofy coed and a hard-boiled semi-intellectual porn actress, and they need a story about a woman "who has a lot of sex as part of her job -- a hooker, or a secretary." Don't know who Andy Dick is, so can't say if either of the guys resembles him. It's not a bad movie. I watched it again 6 or 7 years ago when I came across an off-air dub from HBO, which I probably still have somewhere.