1. Remove the climate control unit.
2. Inspect for continuity between the fan switch terminals using an ohmmeter.


Climate control unit inspection
Defroster switch inspectionMazda 6 Service Manual: Cruise Control (Some models)
With cruise control, you can set and automatically maintain any speed of more
than about 25 km/h (16 mph).
WARNING
Do not use the cruise control under the following conditions: Using
the cruise control under the following conditions is dangerous and could result
in loss of vehicle control.
...
Mazda 6 Service Manual: Battery recharging
Warning
Keep any flammable away from the battery, otherwise evaporated gas
of the battery fluid may catch fire, and cause serious injury or death.
Caution
When removing the battery, disconnect the negative battery cable
first and install it last to prevent damage to electrical compo ...