If connected as specified (one terminal to power, one terminal to Pin 29 of the SV-EMS-220), The External EMS Warning Light should only light when the SV-EMS-220 provides a connection to Ground during an EMS Warning.
Also puzzling is that if the SV-EMS-220 / SkyView is providing a connection to Ground, the light should come on and stay on until ACKed.
As always, check connections.
Intermittent connection to power probably wouldn't cause it to flash, so look for any way that there could be an intermittent connection to Ground.
