Matt,
I added an LED indicator back in 2016. I didn't want to cut any wires or punch holes for the LED so i made a small bracket for a small green LED shown just under the fog light switch in the first pic:
I tied into the vacuum solenoid valve under plenum as Phil had suggested in an earlier post. This pic shows the valve on the right side with the 2 pinkish wires connected. You will need to tie into those 2 wires. I don't have a pic with my tie-in.
A few months later, I removed the secondary system and installed Marc's calibration to remove the secondary controls but I left the LED connected anyways just for fun. This pic shows the 2 LED wires running from the vac solenoid connector to LED.
I needed a couple of resistors and made up this simple circuit. You won't need R2 if you keep the vac solenoid connected but in my case, since I removed the secondaries and vac solenoid, I needed to add R2 because the LED would still glow faintly even when secondaries are off. This is due to the ECM output leakage even when off still able to drive the miniscule load of the LED.
In my case with Marc's secondary delete chip the LED is only off at idle. As soon as i touch the gas pedal the LED turns on (and so do the secondary injectors.) In the end, not much use for me but was informative when I had the secondaries working.