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Do you have check engine light? If not you have an electrical problem within the speedometer assembly.
The entire dash electrical pod as an assembly may be loose . I would take it out and inspect it and the contacts where it plugs in.
I believe the fuse that protects the speedometer also protects all the gauges in the dash cluster, so if that fuse was burned out all the gauges would be dead. If only the speedometer is not working I would suspect the speed sensor in the transmission to at fault, if the wiring checks okay.
The power or ground connections to the instrument cluster could be bad. Check the fuses, then the wiring to the dash to find a common problem for the heat, gauges and speedometer.
Check the fuse.
Some times a bad battery or a bad ground can do this.
Nothing
This happened once with my S-10. What I found was that the wiring for the license plate lights, that were actually rusted out, was grounding causing the the fuse to trip. I just took some electrical tape and fixed the spot. Hope that helps.
Try changing the headlight switch.
you have a shorting wire someplace
If this is an electronic unit, the heat generated after the lights are on for one half hour, is affecting the intergrity of the electronic circuit board.