Seeing you are new to the Reatta forums allow me to let you in on a secret [and this isn't directed at just you but all the "Newbies" that come on line here].
5 years ago you might have gotten 10 guys to give you responses but they are all gone now. Even the Reatta Technical Advisor on the AACA Reatta Forum hardly offers advice. There are really only three of us left. 2seater, Ronnie and myself. So if you get a response at all [on one site or the other] consider it "Golden" and start in.
This isn't a Mustang, Corvette, or Camaro forum where hundreds of thousands of cars were sold, the Reatta sold less then 22,000 over 4 years so parts and advice is getting scarce.
So allow me to add to what 2seater already told you;
The top screw to remove the controller is the hardest one to remove and when you reinstall the unit leave that one out, it's really not needed. The cardboard that 2seater refers to is like the cardboard on a scratch pad, it's that high tech and is held in place by strapping tape. Once that is removed you will see that the motor is right there waiting for you to remove it. I commend you for figuring out what is wrong and actually getting a replacement part.
Jim Finn has a number of these for sale, pretested and he is reasonable in pricing. He is an excellent source for parts.
There are a lot of other parts that you might want to look at obtaining if you plan on keeping your Reatta for any length of time. The 1990 Reatta only, one year only, headlight switch, instrument cluster [88-92 Reatta and Riv], teves pump and motor, teves pressure switch and ball [89-90 on the pump/motor, 88-90 switch and ball], struts/strut mounts both front and rear, ABS sensor leads.
And for other year owners, the Reatta only headlight switch [88-89], CRT and instrument cluster [88-89 Reatta and Riviera].