Abstract: An amplifier unit, adapted for use in a vehicle and for enhancing output signals of the vehicle sound system includes a housing provided with an opening for accessing the interior of the housing, an audio signal input attached to the housing, electric circuitry mounted within the housing which forms an apprible amplifier with basic operating characteristics for amplifying received signals, a connection device for optionally receiving module units having additional circuitry within the housing and for electrically connecting the additional circuitry with the basic electric circuitry to modify the basic operating characteristics of the amplifier, and an amplified signal output. This amplifier unit enables a consumer to buy a basic operable amplifier to which he can add special functions enabling him to build a different amplifier from his basic model.