It's actually pretty easy. I did it several years ago in my El Camino. Easiest step would be to find a visor that would fit the Fiero. All you need is 12v and ground to the visor. I went the harder way, and modified my Elky visor ( I was recovering it anyway). To do this you buy a used Homelink Module on Ebay for about $20 ( inflation- used to be $10!). Get one that is "Rolling Code". It's usually mentioned in the description, but most newer ones are this style. Color is not important, since you can dye them using SEM or even Duplicolor, but if you can get the right color to start, so much the better. Next step is to get power and ground to the visor- both are available in the overhead Dome Light console, so that part is easy. You want constant power for the module. If you want to get fancy, go to the pick and pull, and find a lighted visor. Grab the connector with some pigtail. they are very small, and work well in the small confines of the headliner. The rest is all in modifying the "backbone" of the visor so that the homelink will fit, padding it just a little ( I used 1/8" foam), and then recovering.
Here's a link for a general write up on recovering visors .