If you're going to use a wiper motor, I suggest you use a 12V power supply to maintain the torque and also use a PWM controller like this one - http://store.qkits.com/moreinfo.cfm/MX033 to fine tune the speed. On my first try, I used a wiper motor and a 5V power supply because I had it. Even wired to the slowest speed, it was too fast. I then got the PWM and was able to get the speed down but the 5V power supply lacked the power to operate smoothly. It would be better with the 12V power supply but if you have to go out and buy everything, you may as well get a proper motor that is rated for continuous service and runs around 5RPM.
I ended up replacing the wiper motor in mine and am really happy with the action now.