What fuses have you changed, were the blown
MultiECUscan let's you log the ECU battery voltage, what's it saying at ignition on and while running
Here's mine logged
View attachment 460410
Check the auxiliary belt
Check the battery posts for corrosion
Check the earth lead are the other light correct especially the oil, should be on at ignition on and stay on while the engine starts cranking and go off after a few seconds or when the engine fires up
Check for loose wires to the alternator and from the starter
Then your down to checking with a multimeter
The light is computer controlled and only come on when it detects a fault at startup or senses less than 5.5V
Output should be 14.1V at idle with no accessories or lights on