1995 Buick Roadmaster

Many auto parts stores will check your charging system for free. That will determine if you need a battery or maybe the alternator is not putting out enough. When the A/c is on, can you change the output from floor, dash, and defrost? If not, you can have a vacuum leak

Agree on the auto parts stores. They can test the battery and charging system. With the engine running, the volts should read 13 - 13.5 or so. I had a 95 Roadmaster and last year had the heater core replaced. When they were done, the air flow would not change from defrost to the floor. It was caused by a pinched vacuum hose when they re-assembled the dash board. If the air controls do not change at all, it is likely a vacuum problem (assuming the buttons are showing that you pressed them to change).