the 89FB is not a serial number its a part number. They did change a bit as subtle changes were made to radios in the same series (e.g. the RDS2006 model).
I have a couple that are in the format
[model number] [serial number] [part number],
but the boxes say which is the serial number in smaller letters.
Serial numbers sometimes begin with an L or early ones with a C, but I think they were 3 or 4 digit serial numbers. The ones I have have an M and 6 digits.
Sorry I cannot help with a decode but hope the info is of some use.
Do a search on google for "entering ford radio codes" and one result is as below
http://www.decodemyradio.co.uk/entry.php
It could help you decide if you have an early or a late 2005 model by the number of flashing dashes on the display.
If you can't get it decoded I have a couple of rds2006's that I have been meaning to put on ebay, but I think the 2005 only had 2 channels not 4 like the 2006: the 2006's will work fine with just 2 but you won't get the full functionality of the radio itself. On the other hand, if for some reason the car wiring has all 8 wires in the speaker connector you may have 2 unused speakers in the car that you can bring to life and enjoy more decibels!