i just bought this car & the instrument cluster isn't working. its not the fuses

you either have a bad connection at the cluster or the cluster is bad

if you are driving during the day, you will see how fast you are going on your dashboard with the speedometer. However, at night you can control how much light the dashboard should illuminate so that you can see the dashboard panel, such as the speedometer and how much you have in your gas tank etc. I would also take your car to a GM/Cadillac dealership and see what they can tell you. Also have them run a diagnostic test to see what the problem could be.