I was asking this already in MS-Forum, but without success. Maybe somebody here knows something?

I'm using the Microsoft.Phone.Maps.Controls.Map in WP8 and want to add my own TileSource to the Map with map-tiles stored in isolated storage. So I wrote a new TileSource-class, overriding GetUri. But now is the problem, how to build the Uri which is supposed to be returned by GetUri so that its pointing at the local tiles in iso-storage?
I tried this, but without success:
Return New Uri("isostore://mytiles/23/34/12.png")
Thanks, Tom