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Yes the bulb is required for the alternator to start it's charging state.....
also.....on 944s (late only iirc)) the alarm brain is in the passenger footwell, that circuit loops through it......
On cars without an alarm it still has the alarm harness which has a jumper plug in place of it.....
I have seen numerous people remove their alarms then have a no charge situation for this reason.
I have also done many conversions with gauges etc....and without that little bulb working the alternator will not charge.

Also the light and volt gauge run off of 2 different circuits...