I've been struggling with the silent uninstaller for a while, and finally came to a solution that works for me in most cases, both for InstallShield v6 and v7.
1. First (as it was mentioned above), you have to generate an InstallShield Response file (e.g. uninstall.iss). In order to do that you have to launch your setup.exe with parameters:
> setup.exe -x -r -f1"C:\Your\Installer\Location\uninstall.iss"
This will go through the normal uninstall wizard and generate a Response file for you: uninstall.iss
2. Then, before trying your silent uninstaller, I guess, you should re-install the software.
3. And finally, run your silent uninstaller playing back the recently generated Response file:
> setup.exe -x -s -l0x9 -ARP -f1"C:\Your\Installer\Location\uninstall.iss"
That's it.
Now, a few important notes:
Note 1: I'm working with a 3-rd party installation package that I didn't build myself.
Note 2: I use dashes (-) instead of slashes (/) to define parameters. For some reason it doesn't work with slashes for me. Weird but true.
Note 3: The -ARP and -l switches are required for some installation packages to manage the software removal from the Add/Remove Programs list and to preset the default input language accordingly.
Successful silent uninstallation is all about the correct parameters!
So keep exploring, the correct parameters vary depending on a specific package and installer version.
I hope my input was helpful.