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I would check the headlight switch for getting hot, check grounds, and check the connectors to the back of the cluster. The headlights use electricity, and also turn on the cluster lights. If there was a bad ground or poor connection to the cluster, it might make the speedometer become the pathway to ground to run the lights... or the radio .. or the blower motor .. or wipers .. Once connections are un-made for any reason, weird stuff can happen.
tom
what tomw said,, and check your grounds to the cab, i had a lot of problems with my 99 ranger because of that also your altenator could be getting weak or going bad.
Problem solved. Found wire under dash the one that goes to the parking brake for the light had grounded out. It had been cut into in the past. Whoever patched wire did a crappy tape job. Respliced and heat shrink. Everything is well now. Oh and replaced shot fuse. Thanks for responses.
OK, good find, fix, feedback & to hear all is well & your on the road again without the speed-o acting out. First I've heard of this type of problem messing with the speed-o.