My recollection is this is perfectly normal. With the headlights on and the
car cruising, adding further load (like defogger, A/C, interior fan) will
lower the voltage at the alternator's regulator, and so dim the lights.

For reinforcement of this, see the service manual page for testing 1995-97
Civic's charging system, at the link below. Note that the voltage "checks
sat" if, with the engine running and the headlights on, and then the
interior fan and defogger is added, it drops from over about 13.6 V to under
13 V.

http://media.honda.co.uk/car/owner/m...pdf/23-a96.pdf

"Ron M." <nobody@nowhere.net> wrote
> Whenever the alternator of my '98 Accord sedan gets an additional load put
> on it, the headlights of the car temporarily dim to some degree. This
> additional loading seems to take place whenever I turn on the rear window
> defogger, A/C, or fan. The alternator belt isn't very old at all and it
> isn't loose either, so I am suspecting that the alternator itself could be
> on its last leg.
>
> Other than the alternator belt or alternator itself mentioned above, is
> there anything else that could be causing this dimming of the headlights
> whenever HVAC controls are turned on?
>
> Thanks in advance for any help/advice that you may have to offer me on
> this one.