your windshield seal could be bad or you may have a bad heater core if it is leaking bad you'll know soon enough, its the heater core!. Mines doing the same thing and its the heater core. The bad news is that the part is only 75.-100. bucks but the labor will eat you alive.
The 1991 Cadillac Deville oxygen sensors are located on the firewall and the air cleaner housing. The firewall oxygen sensor will be on the drivers side. The air cleaner oxygen sensor will be on the back of the air cleaner housing.
1991 Seville VATs module is located in the trunk midway between the rear deck speakers. Access by pulling down the upholstered rear deck cover in the trunk. Module is a black box about 1 inch by 3 inches by 5 inches and has many wires attached by connectors.
Begin by removing the power steering pump belt from your 1991 Cadillac power steering pump pulley. Remove the power steering pump pulley retaining bolt. The retaining bolt will be in the center of the pulley.