I had a 93 Buick LeSaber that I replaced the ECM in. Before you buy a new ECM and start taking your car apart you need to buy a service manual. For a 91 Trofeo I would suggest a Chilton's manual. You don't need to spend big bucks to get a factory service manual for a car that is your daily driver, but a Chilton's manual will have the ECM replacement procedures you are looking for.
When I did the ECM on my Buick it was due to a Quad Driver Module failure (part of the ECM that actually went bad). I asked around a dealerships and auto shops and found out it was much cheaper to do the repair myself than having someone else do it. I think I spent about $120 on a new ECM from the auto parts store and took about 2 hours to take out the old one and install the new one. It was behind the glove box in my Buick so I didn't have to take many parts out of the car.
Ultimately, if you decide to replace your ECM you need to use an anti ESD (electro static discharge) device when you handle the new one. This might be the hardest part of the job because they might be hard to come buy unless you have a friend that works in electronics. An anti ESD device is just a braclet that attaches to ground either through an anti static mat or straight to a ground point on your car. Whatever you do DON'T handle a new ECM without one of these. You will be throwing away your money!
