Subaru 1998 Forester Owner's Manual page 110

Indicator light
HS0063
To turn on the outside mirror defogger, push the switch. To turn it off,
push the switch again.
The indicator light located on the switch is on while the outside mirror
defogger is operation. The defogger will automatically shut off after
about 15 minutes. If the mirror clears before this time, push the switch
to turn it off. It also turns off when the ignition switch is turned to the
"ACC" or "LOCK" position. If defrosting or defogging is desired when
you restart your vehicle, you have to push the switch to turn it on
again.
NOTE
While the outside mirror defogger is operating, the rear window
defogger also operates
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