I'm not sure that light has anything to do with the voltmeter, other than sharing space. Here's the relevant page out of the 968 owner's manual. I don't think this is any different from the late 944. If it does not light up, you are supposed to contact your dealer
My understanding is that the bulb is part of the exciter circuit for the alternator; unlike a generator which is simply a DC device and will produce power whenever it is turned, an alternator needs to have an excitation current to induce a magnetic field. If the bulb is broken, then the alternator never establishes the excitation field and doesn't charge.
Furthurmore; from wikipedia on alternators:
"
The field windings are supplied power from the battery via the ignition switch and regulator. A parallel circuit supplies the "charge" warning indicator and is earthed via the regulator.(which is why the indicator is on when the ignition is on but the engine is not running). Once the engine is running and the alternator is generating power, a diode feeds the field current from the alternator main output equalizing the voltage across the warning indicator which goes off. The wire supplying the field current is often referred to as the "exciter" wire. The drawback of this arrangement is that if the warning lamp burns out or the "exciter" wire is disconnected, no current reaches the field windings and the alternator will not generate power."