Use DVOM to check continuity. ON RX7, check between control unit harness connector terminal "D" and JC-02 connector. See Fig 1
and Fig 3
or Fig 2
and Fig 4
. On 929, check between control unit harness connector terminal "T" and JC-02 connector. If continuity is not present, repair harness.
Fig 1: Identifying Control Unit Harness Connector Terminals
Courtesy of MAZDA MOTORS CORP.
Fig 2: Identifying Control Unit Harness Connector Terminals