Craig,
The tail light is fed from a separate contact of the ignition switch, it's that way so when you put it in Park just the tail light and the small lamp in the headlight are on, I think this wiring must be a legal requirement, most bikes seem to be that way.
http://www.thisoldtractor.com/guzzi007/schematics/2005_Breva_1100.gifIf you look at the schematic there's a yellow wire from fuse B that feeds the ignition switch
When you modified the yellow at the start relay you should have left it alive to feed the ignition switch tail light contact.
Fuse A & B are fed from the switch via Green/Black wire, check those 2 fuse holders for 12V with the key ON, if nothing try it with the switch in Park and get back to us
Please describe your Startus Interuptus fix just in case there's something wrong there, did you cut and re-power the yellow wire?