I have an Autometer shift light (the one which uses "pills") and it is connected via a separate RPM module. I bought my RPM module from Paul's High Performance... click on the following link to Paul's web site, then click on "Mustang" on the left and you will see the "Tach Interface Module" there directly on top.
This is needed to interface with the car's electronics and to provide input to the shift light. There are probably other devices out there similar to this one which work equally well, and may even cost less?? But this one I know works, as I have it installed in my car.
I bought my shift light from Jegs online... click on the following link to go directly to the shift light I bought...
The pills are required to tell the shift light at what RPM you wish to trigger on. You will see on the web site that they come in sets. I have the 6600 pill in mine. So, my light comes on if/when I hit 6600 RPMs. I think I am going to swap pills in favor of the 6400 one though, as I find I hit the limiter too often, especially in first and second gears.
Let me know if you need more information or clarification on this.
By the way, I also have the Autometer mini-Pro lights installed in my car along with the shift light... one for high water temp, and one for low oil pressure. This allows me to concentrate more on my driving at the track and not watch the gauges as much. If something bad happens (like a blown oil pump - YIKES!! ) the warning light will come on bright red to tell me it is time to shut her down before real damage occurs.
I finally got my shift light from RaptorPerformance installed.$75 Apex Motorsports ran the wires for power down to the fuse box, and the coil on plug lead to an underhood test point. I decided to put it over by the A pillar using some Velcro in case I want to relocate it later. No drilling! The yellow LED's are plenty bright, so I don't think seeing it in the daytime will be an issue.
I've got it set to 6200 RPM at the moment, but it uses little rotary switches so changing it later will be no problem. No tach driver required.
I have mine installed in the same general location, but actually punched a hole through the dash (It's not stock any more anyway!) and it looks good there and I can see it well from the drivers position.
I have a 6600 pill on mine, and that seems to work well. I figure I need a second or so to see and react, to make sure I don't tap the rev limiter which is right at 7K on my car.