I want an AEM digital dash for a car with a factory ECU (e.g. BMW E46 M3, Siemens ECU). What do I need?

With a factory ECU that the dash cannot read directly, you need not only the display but also the sensors you want to monitor, connected to the dash inputs or to AEM CAN modules. For example, to monitor AFR you need an AEM 30-0310 wideband controller.