I may be way off base here, but take a look at the wiring harness and the wiring diagram in the HD Service Manual. I'm looking at the manual for an '07 Dyna but the Softail probably is the same.
The data link/bus wire is light green/violet (lgn/v) and comes in on #2 of the plug to the speedometer. It is also #2 on the tachometer plug, if a separate unit, and they are the same data link. Check the signal there, at the plug. I don't know what the signal looks like, but it should match at the ECM, TSSM, and the speedo. The wire colors should be the same throughout; #7 on the plug to the main harness, #3 on the TSSM, and #5 on the ECM. Again, the Softail may be different but this is what the Dyna diagram shows.
Might want to check with the HD parts people to see if there was any change in the speedometer or harness between these years. One of our electrical experts should be checking in also.
Let us know what you find out, and how you fixed it.
.