Here's a couple of variations on the Alternator Charge LED theme.
The circuit on the left is the most reliable, with least chance of the LED ever blowing due to overvoltage or reverse voltages. The bridge rectifier can be any cheap 200v 2A one you find lying around, or cheapest you can buy. You may need to adjust the resistor values slightly, as I just reduced their values slightly to allow for voltage drop across the bridge (diodes). With this circuit it doesn't matter which connection goes to IGN+ or Alternator, since it's using a bridge rectifier.
The circuit on the right should be reliable in most cases and may well outlast your alternator, even though it's the cheaper nastier version. Note that it definitely matters which connection goes to IGN+ and alternator. Adjust the 560R series resistor to suit the LED or change brightness. A super-bright 10mm LED may need a lower value resistor, for example.
In the relevant circuits, R1 and R3 control the LED brightness, or should be adjusted to suit the required LED current (usually about 20mA). R2 and R4 need to be 5watt and simulates the current limit (or drain) of a standard dash light bulb so the alternator works correctly.
{edited spelling}
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