Instrument panel

Disconnect the negative battery cable. (See BATTERY REMOVAL/INSTALLATION [LF] )
Remove the scuff plate. (See SCUFF PLATE REMOVAL/INSTALLATION )
Remove the side panel. (See SIDE PANEL REMOVAL/INSTALLATION )
Partially peel back the floor covering so that the foot support can be removed.
Remove the foot support.
Disconnect the connector, then remove the screws.
Remove the keyless antenna.

Fig 1: Identifying Keyless Antenna And Foot Support
Courtesy of MAZDA MOTORS CORP.
Install in the reverse order of removal.