Fan speed doesn’t change with max ac feature, that closes a fresh air door to recirculate mode. While it may sound faster it’s actually because the fresh air is shut off. As far as resistor, it is mounted on the air box where air traveling across will cool it, generally right below the glove box area. High speed is usually a direct 12v side of the resistor, then each speed becomes another side of the resistor . Check for 12v at the blower motor on high, then check the other speeds, you should notice staggering voltage numbers. If you don’t have 12 check the resistor for damage, the steel coils could rot off, or burnt connectors at the resistor, then also test the motor with a known 12v jumper to verify it works. If you have 12 volts to the resistor and the motor works good with 12v jumper then check for an open with a good voltmeter on the resistor, next you will need to test the switch for current going thru once again 12 volts then progressively goes down. A good quality volt meter, factory service manual and jumper set will be your best friend always find away to fuse it somewhere on the positive side. If your really confident a power probe is sweet but you must be very very careful.. best of luck
