9 times out of 10 it is the speedo cable rather than the sending unit.
Here is a link to a page that shows how to get diagnostics for the trans.
http://www.cygnusx1.net/Supra/TechTi...ror_codes.aspx
Here is a link to a wright up on replacing the speedo cable.
http://supras.com/06/techcenter/display.php?QID=29
I've got no clue about the horn problem though.