check the fuses under the dash 1st, if all is ok, then check the wires that go to the v.s.s. (vehicle speed sensor). i have seen the wires melted together b4... local dealer should carry the pigtail in stock...
theres more that it could be, but that would be a good place to start.
and b4 i forget, why did u replace it? was the speedo already not working? was it leaking? more info please.