I too, was under impression that you need a GPS data source for ADS-B installation. I'm talking about certification, not failure/MEL cases. I know that if GPS is unavailable, an ADS-B receiver will transmit FMC position data (coming from IRS or radio update), but it's accuracy would be flagged low, so ATC would not use it anyway, thus kind of negates the whole point of having ADS-B.
DaveReidUK, do you have any reference about ADS-B installation without GPS?