You have a very common problem on these cars, the (blower control module.) It is located on top of the fan box under the fuse and relay section on the firewall. It has two wiring connectors connected to it and usually three screws holding it in the box. You can get a new one from any dealer for around $150.00. It is such a common problem on Buicks and other GM cars with auto climate control, that the dealers usually keep a good supply on hand. You can also check your local salvage yard, (about $50.00) they usually have some at the front counter. Be careful to insist on the same part number as the one you have removed if buying from a salvage yard. (The part number will be on the module). GM made four or five different controllers that look the same, however do not have the same number of wires or the same functions. You do not have a relay for the blower fan because the controller sends variable voltage to the fan to control the speed. Usually the resistors fail in the module and the default voltage is 12V direct to the blower unswitched, therefore your blower runs all the time unless you pull the fuse, disconnect the battery, or pull the plug on the fan itself.
