The non-fixed version of i6comp, as previously stated, will NOT work with newer InstallShield versions, such as those found in GTA3, etc.
The fixed version WILL work most of the time, however, I did find that the fixed version produced some random errors at times, i.e. while trying to pass the cab name value and when trying to access the data2.cab, as well. At least, this was so on my tests.
The version named i6comp02.zip, located
HERE did NOT produce the same random errors and worked fine with the GTA3 data*.cab and wrote the data1.hdr files correctly, as well.