Brake lights on some vehicles just do not work with the ignition off. I believe this is a feature to save the battery in case there is a fault in the brake system, but I do not know. My car’s brake lights do not turn on when I have the key off either.
LED 1 indicates that the system is receiving power, so that would match with your computer. LED 2 is that the logic board is detecting proper voltage from the power supply. It also is used to show if the logic board regulators are working properly. Here are some things to try if the second LED is not working or flashing: • Verify power button connection to logic board • Verify power button functionality • Verify cable connection between power supply and logic board • Verify power supply • Verify logic board Source: iMac Service Manual: https://data2.manualslib.com/pdf3/63/624...-apple/imac.pdf?3f0d9975d1dc5082e9f6da0a4...
Check the ram by opening it and making sure they are seated correctly. Next, turn it on and shine a flashlight on the display. If you can see something on it, then the backlight is bad. If you can't see anything, then check the connector for the display. Or the display could be bad.
Check to make sure that the backlight cable is plugged in all the way and free of dust. Make sure the connector is seated correctly and locked in place. It could also be that your backlight inverter is going bad and will need to be replaced.