Sounds like it could be a display fuse. Is your unit under warranty? It's a fiddly surface mount component, not for the faint hearted or the ham fisted! Probably worth dropping David a line. You say you've taken your unit apart, did you disconnect you display board? If so, were you super careful when putting it back? Some of the early backlit button hackers accidentally put the display connector back off by one pin therefore blowing the fuse.
Support contact details are in the riocar.org FAQ. Hope everything works out...