Can you identify the diode / resistor in the alternator, or is that all just a sealed block?
Your cheapest solution might be to put something like a 100R 10W resistor, typically metal clad and mounted on some sheet metal as a heatsink, and a 1-3A rectifier externally between the IG and L lines to essentially "parallel" the internal diode/resistor which presumably have an open fault.
This sort of idea:
You could use a 5W 24V bulb instead, but then you ought to make the rectifier 3A to cope with switch-on surges. Cost difference is minor, something like this would do:
Your cheapest solution might be to put something like a 100R 10W resistor, typically metal clad and mounted on some sheet metal as a heatsink, and a 1-3A rectifier externally between the IG and L lines to essentially "parallel" the internal diode/resistor which presumably have an open fault.
This sort of idea:
You could use a 5W 24V bulb instead, but then you ought to make the rectifier 3A to cope with switch-on surges. Cost difference is minor, something like this would do:
onsemi 200V 3A, Rectifier Diode, 2-Pin DO-201AD 1N5402G | RS
uk.rs-online.com