location of speed sensor

It's in the transmission. However, if your speedometer isn't working it's likely an instrument cluster problem. This is very common on the 03-06 Trailblazers. The cause is the motors behind the gauges are failing, they all go out eventually. You can get it repaired here - www.drspeedometer.com and there's a DIY guide here - www.howtorepairmyinstrumentcluster.com