For anyone looking to correct this problem or change how their alternator is energized, the bulb is probably the easier, cheaper option. Not only can you make use of its function as an indicator, its also much simpler to connect a readily available incandescent 12 V bulb in the circuit than to source an appropriate power resistor. If the resistance of the bulb used is indeed ~5 ohms, this provides a Current = Voltage/Resistance = ~12 V/5 ohms = ~2.4 Amp signal to the windings that will produce a much stronger magnetic field. This will likely lead to the alternator powering up faster than if provided with a lower current signal although it really depends on the actual alternator circuit involved. In order to provide an equivalent signal, a power resistor of 5 ohms rated for at least Power = VoltagexCurrent = 12 Vx2.4 A = 28.8 W is needed. Such a resistor would likely cost more and be more difficult to obtain than a simple incandescent indicator bulb but without the added ability to use it as an indicator.
However, as Steve and Jack have pointed out above, a 2.4 Amp signal may not be necessary for the alternator to turn on properly. Steve seems to be utilizing a much lower current signal of ~12 V/85 ohms = ~0.1412 A with a 3 W resistor. This seems to indicate that perhaps only a small current signal is needed as Steve's is 0.1412 A / 2.4 A = 0.0583 or about 6% of the one Mark is using for his.
It is a bit beyond the scope of this thread to discuss why the signal is needed and how large it should be but the most important thing to understand is that the L terminal must be seeing at least some current signal for the alternator to begin charging.
