Two weeks ago I changed my dashboard cluster, so now I'm going to explain how I did it so if anyone has to replace the cluster they can follow my steps. It was fun thats for sure.
The dash cluster in 87-91 are pretty sophiscated for their time. Its one unit, and looks like a circuit board on the backside, be careful with it. I nicknamed "the plug" but we'll get to that later.
Disconnect the negative battery cable. Put the tilt steering all the down as far as it will go, and if you have the colum shifter, put the car in 1st. Be sure the Parking Brake is on. After the 1989 model year, GM and most other car companies introduced shift interlock. Basically the vehicle cannot be put into gear unless the car is on and the brake depressed. If you own a 90 or 91 Bonneville with a column shifter, don't force the gears into first or any other gear, it simply won't go. Shift interlock is a safety mechanism. Its there for a reason. However 87-89, make sure the parking brake is on and you're safe to shift. Remember you are only doing this to simplify the process of changing the cluster
Remove the front vent grille and speaker covers (if equiped, the SE and SSE for sure have them). You can leave the front speakers in. Twist out the Twilight Sentinel sensor. Most of us know how cheap and flimpsy those vents are and chances are half of it is popped out anway. You'll find 3 screws, take them out. You will find that they are pretty long.
Take the plastic wood trim and CAREFULLY pry it out of place. You will see more screws on the left and right side of the cluster, take those out and remove the vinyl top cover (You should have 5 screws that hold the top cover down).
Take off the steering column cover on the underside of the steering colum.
3 screws hold on the dash cluster. Unscrew the bolts and CAREFULLY pull out the cluster. On SSE models, the compass and DIC will have additional wires and plugs, LE and SE owners, don't worry, give the cluster a pull and the cluster wil come out. Basically its a plug, with multi-prong pins at the center of the unit on the backside. Check your new or replacement cluster for adaquate number of lights. There should be 194'* all over the place on wireless light fixtures. The cluster powers the lights via the circuit board mess that the cluster happens to be.
While you are in there, give the underside of the dash a cleaning.
You'll notice on your gear indicators at the bottom of the cluster a high-tensive wire with a metal clip. This wire operates on a prehistoric spring system. Most column shifter cars will force you to remove the metal plate under the steering column. I'm not sure what it is called but is a metal plate held in with screws. Its easy enough to take off but don't lose the screws.
The metal clip/wire simply slides into the steering column and when you change gears the spring inside the cluster pulls on the wire and tells you what gear you are in...pretty clever huh?
Once the cluster is put into place and hooked up, reconnect the battery and take your car for a drive and make sure all the gages, light and odometer work. Put back on the metal plate from underneath the column. Screw in the dash top cover, put the speaker caps on, reconnect your twilight sentinel to the grill cover. Screw down, careful snap back into place the plastic trim piece and finish it off by re-attaching the steering column cover plate.
Its really easy to do, so don't go pay someone to do it. I'm a mechanical idiot and I did it in 1.5 hrs...and 2 beers...