your ISC likely pushes out J1708 or J1939 Raw data... a scangauge D can read thew data and error codes.. its a paid device..
ive done quite a bit with ECM hacking on the 1708 and 1939 streams of my navistar using devices from bb-electronics which do the heavy hitting of the protocol.. if your systems is J1708 .. (1999 is a transition year .. between 1708 and 1939 ... do you have the 6 pin or 9 pin port??)..
then you need an ELM 325, the ELM327 cannot talk J1708 / 1587.
I am writing most of my own software for the digital dashes now on my red bus.. some of it is commercial at this point but others is my own home brew.. (as I have time).. I used a raspberry pi 3 with the bb-electronics device and read / translate the 2 data busses in my engine... I have both 1708 and 1939 on my T-444E engine...
-Christopher