Resistor

[n86121]

As I recall from 20 years ago,

the alternator out light leads:
one goes to 24 volts,
the other goes to the alternator output line, and this resistor takes that line to ground.

If the alternator is putting out 24 volts both sides of lamp at 24 volts, no glow

If alternator not outputting 24 volts, this pulls lamp down to ground, lamp on.

From Facebook info, maybe 220 ohm resistor.

Dead short would be under 4 watts, so 5 watt rating should do the trick.