With the Mazda box, the filler / level plug is on the LH side and the bolt has a square head, not to be mistaken for the hex head bolt that is located directly below it. If you pull back the carpet in the front of the passenger footwell you should find a large rubber grommet that will give you direct access. Whilst it is not a routine service item, the Mazda box has a separate top up for the selector turret which is accessed by removing the gearshift. This seldom needs attention but worth checking if you ever have the trim cover off the driveline tunnel.