Well, I was thinking if you are going to take signals straight out if the ECU you might want to put something between the ECU and the LED - sort of like a relay - so that the LED circuit and the ECU gubbins remain seperated.
However, just thinking about it a bit more, if you took the signal from the "valve end" (probably a higher current) then I reckon you could tap off about 20mA to drive the LED without messing anything up.
Only thing is, IIRC the Supra has all its switches on the earth side of the circuit, so it may take some head scratching to hook the LED into the loom.
Possibly a question for Mr. Betts.