How do I get the Mbox button to flash for a secondary Group Mailbox. Apparently Mbox0 does automatically but how do I get say Mbox8 to flash a light when there is a VM in that Mailbox?

A group mailbox may not light a light if the system doesn't have a valid number to dial, like an extension number, to light the light.

What voicemail/ system type are you working with?