ok, so just looking at my post I spotted mistakes..
This actually returns a success:
Execute-Process "C:\Windows\System32\RunDll32.exe -Parameters ""C:\PROGRA~2\COMMON~1\INSTAL~1\PROFES~1\RunTime\10\00\Intel32\Ctor.dll,LaunchSetup" "C:\Program Files (x86)\InstallShield Installation Information{0E6C2C52-A7FF-47DC-AD2A-200325805041}\setup.exe"" -s -f1""C:\setup.iss"
But it doesn't actually do anything..:-(
This actually returns a success:
Execute-Process "C:\Windows\System32\RunDll32.exe -Parameters ""C:\PROGRA~2\COMMON~1\INSTAL~1\PROFES~1\RunTime\10\00\Intel32\Ctor.dll,LaunchSetup" "C:\Program Files (x86)\InstallShield Installation Information{0E6C2C52-A7FF-47DC-AD2A-200325805041}\setup.exe"" -s -f1""C:\setup.iss"
But it doesn't actually do anything..:-(