mate, try posting in the ICE section as well.
which line is it you are checking, the switched live or the permanent live?
if you are not getting power to the unit then the fuse in your headunit is too far down the line to be making a difference, so it certainly can't be that.
it will almost certainly be a fuse that has gone, do all the interior lights etc still work? it could well be that the radio is on whats called a room fuse which supplies all manner of interior functions.
check the fuses in the glove box panel, the fuses by your right kneee and the fuses just abouve that.
there are also some fuses relays etc around the battery.
Out of interest what caused you to be tracking the wiring back for your head unit?
1999 156 1.8TS Sport Pack 3
2001 166 3.0 V6 24V Super
2003 156 GTA 3.2 V6 24V
2005 156 1.9 JTDM-Jet Sport
2008 159 1.9 JTDM Lusso
1997 GTV 2.0TS Lusso
2001 147 1.6TS Lusso