Boot with no startup sequence to get in to a PAL/NTSC screenmode from the Amiga's native output. Then run P96Mode (in prefs) to configure a working screenmode. You will know it is working because the testcard will be visible on your VGA monitor. Then go in to screenmode prefs and select this screenmode, and save it. Then reboot. And you should see Workbench on your VGA monitor.
--
moto