My 2004 trailblazer speedometer will not go to 0 nor will it track the speed. I have changed the speed sensor on the transmission but it still is not working. What is the next step to resovle the problem?

Use a test light or multi-meter to check for a blown fuse in the under-dash fuse/relay box. If this is not the cause, then you need to find out the various VSS Input Tests (at the harness side of the connector). There should be tests for checking continuity to ground and for checking voltage to ground for the pins in the VSS connector.

I would also get a scanner and check for a VSS-related code being thrown.