Please refer to section 126.96.36.199 on page 60, table 30, pin 2 and pin 4 for the front panel POWER LED, and section 188.8.131.52.3.
For the alternate front panel power LED, refer to section 184.108.40.206.5.
For the location ot the front panel header, refer to section 2.2.2, page 51, figure 11, location R, and location Q for the alternate.
1.8v sounds correct for the LED voltage. If your current LED does not work when correctly connected to the primary or alternater header, I would try another LED.