I don't think you installed it wrong. But for an extra $60 you would have a brand new speedo cable.
not a half shagged one.Which it sounds like it will have to be replaced soon.
Just unscrew the black surround disconnect all the plugs, then unscrew the instrument cluster 4 screws i think, and just pull it away piss easy.
Brand new $110 i think is cheap considering it will last about 10 years or over.