Dark rides were a serious problem for my DD who was 4 last year on her first trip. The sad thing is that the best rides there are practically all in the "dark"! She screamed bloody murder on all of them with the exception of Its a Small World. The worst rides for her were Pooh, Jungle Cruise (because you go through a dark tunnel/cave!), Peter Pan. And at Epcot, she was a complete wreck in Nemo! I never made it onto to Toy Story Mania or even Buzz Lightyear, and forget about the Haunted Mansion!
I thought just covering her eyes would help, but it didn't. I have been told to try bringing a small flashlight pen, glow sticks too, but knowing my daughter, that wouldn't have helped her. She had up to that point always slept with a night light on and the hallway light on. So she has no experience being in the total darkness. The way the cars move along in the dark is also what scares her because she has no idea which direction she will be heading towards , so it was a combination of the two. We are headed back this year and she says she still doesn't want any dark rides, so we shall see.
My suggestion would be to show some video clips of the dark rides on you tube, practice being in a dark room, try those glow sticks/light pens etc.
We had to keep reminding ourselves that we were in Disney for the girls, and if they didn't like a particular aspect of a ride (too scary, too dark), we had to skip it, even if it was a favourite of mine. We stuck to things that they liked and never forced them to try a ride they didn't want. And if they did go on a ride and discovered they hated it, we of course would never go on it again! Have a great trip!