Most LEDs don't have enough internal resistance. Using LEDs for Canbus systems will sometimes work, for other situations a load resistor will be needed.
I know when I did my gauge cluster LEDs, I had to switch the airbag light back to a regular bulb because it always glowed and set off the indicator fault thinking the bulb was blown.
Using load resistors can become it's own issue, they get hot and aside from lasting longer, defeat the remaining benefits of making the switch to LEDs in the first place.
In the same way the resistor fools the system into thinking a bulb is in use, it can continue to do so even after the LED fails. For example, a turn signal LED out and not get the rapid blink to let you know about it.
I don't know who is in the business these days but there were a couple of sites out there that made LEDs specific for older vehicles that got around some of the issues.
I didn't go that route because it was more expensive than the DIY option.
