sudo /System/Library/CoreServices/RemoteManagement/ARDAgent.app/
Contents/Resources/kickstart -activate -configure -access -on -users
admin -privs -all -restart -agent -menu

This command saved my butt when it came time to install two Xserve’s (headless) as I had no access to a Mini Display Port adapter and Server Assistant “shit the bed.”

It’s worth mentioning you should be ssh’d in as Admin/Administrator.  I speculate this won’t work as root, but I don’t know.