When not using a ignition box, where at the coil do you tie in a aftermarket tach wire.

 

I just installed a shift light which is wired to the car just like a aftermarket tach. The following is copied from the post in the original thread:
I picked up the tach signal inside the passenger side kick panel, blue connector (c230, cavity D, white wire) ref service manual and shbox pic:
http://shbox.com/1/keyless_entry.jpg
Power source from dash mounted fuse panel spare IGN receptacle which is fused from the 'guages' fuse.
http://www.google.com/imgres?q=dash+...w=1009&bih=567
Ground with ring terminal to stud for lower dash panel support by steering column.
All wiring done inside car, didn't have to mess with pcm. It works great, no more looking at the unreliable stock tach when bangin' gears. Just thought I'd post a update.

This info should help you.
Also check out this thread.
https://ls1tech.com/forums/lt1-lt4-m...estions-2.html