The tail light and illumination wiring diagram for an 03 Matrix (same as for your 03 Vibe) was uploaded here somewhere else, and can be found at
http://forums.genvibe.com/files/importe ... -020ti.pdf
I'd probably start by pulling out the trim piece left of the steering column that has the dimmer switch and mirror controls, and disconnect the electrical connector from the back of it, replace the TAIL fuse and see if it blows again. Disconnecting the dimmer will isolate two parts of the circuit powered by the TAIL fuse. If the problem is in that switch, or in one of the things that the dimmer controls, then you'd at least have tail lights ( a critical safety thing)
Your signature says you've replaced the OEM NAV radio. The OEM radio was tied into the TAIL circuit, and the new one may be or may not be, depending on how it was wired up. That may be your only mod that taps the TAIL circuit, so that would make it a suspect. Perhaps whoever did the radio swap didn't properly insulate one of the wires, and its now shorting. Thats a guess, but since its a mod, that makes it suspect.
Seems unlikely, but if the TAIL fuse still blows after you disconnect the dimmer switch, you could try removing the four running lamp bulbs (one in each corner of the car} and see if it continues to blow a new fuse