A system (10) and method (50) of equalizing an audio signal from a factory installed radio (30) using an interface unit (10) between the factory installed radio and a replacement load (34) can include the steps of measuring (52) an existing equalization for the factory installed radio in the interface unit, analyzing (64) the existing equalization to provide a measure of an existing frequency response suited for an old load, generating (66) a substantially inverted frequency response to the existing frequency response, and applying (74) the inverted frequency response to the replacement load to provide a desired response. Such as system can include an interface unit (12) having an automatic input level control unit (14), an analog to digital converter or digitizer (16), a DSP (18) providing an output to a DAC (20), a level booster unit (22) and a processor such as a microcontroller unit (24).