speedometer cable

I have a 1968 coupe. There is a large rubber grommet on the right side of the transmmision tunnel, under the carpet. That gives you access to the cable( sort of) where it enters the transmision. But I think the cable sheath is attached to the transmision, so I have never been able to take it out, just able to withdraw the cable from the sheath. I have a problem with the speedo needle bouncing up and down at low speeds(under 40). The speedo head seems OK. I repaced the cable, but still have problem. Is your problem the same? Also, another question, the under dash hardura on the drivers side(left). I purchased new hardura, but can't figure how it is oriented. Don't know if I got the wrong pieces or if I'm just placing it wrong.