Abstract: A diagnostic device having a diversity processor for individually testing the electrical condition of the connections of individual antennas of a multi-antenna system in a motor vehicle. This device can be coupled to the interfaces of an antenna amplifier containing a diversity system, and such a test can be carried out without any intervention in the circuitry of the receiving installation. The diagnostic device contains a test unit and a control unit. The test unit is designed for generating a test signal that is received by an antenna of the multi-antenna system by a transmitting antenna. The diversity system is acted upon by a control unit having a control signal consisting of an IF-signal and a dc voltage signal. The control signal generated by the control unit by an interference simulator integrated in the control unit is disturbed only when the diversity system is prompted to switch to the next antenna.