My first experience with using a extarnal device was when I was 17, bought a ewe-beaut Pioneer stereo and jury-rigged it into my old lady's Austin 1800..no-go, so switched wires and was ok (didn't realise was +ve earth) but blew a channel ....(Space Oddity on one channel is not so good
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I agree with mainly the last two comments that so long as this kind of speedo is electrically isolated from the frame it'll work, and in the case of the LED light, depending on the if base is isolated like a P43t, the wires should be able to connect as as normal, as the dipping wires only run through the switch and don't earth, and horn doesn't (generally) matter- but well leave that out- and lighting, these should be debated in another arg-thread