This morning on the way to work I rolled my window down and it didn't come all the way back up. When I let go of the power window control the window just went right back down. I eventually got it all the way back up after about 10 tries of rolling it up. I have noticed that over the last few months the speed and power in which the window came up has been decreasing. I'm thinking that the motor is shot. How difficult and expensive are they to replace?