speedo cable replacement: not sure about the other end but getting to the top end is easy.
5 screws holding the dash front on (2 below the steering column, 1 each side, 3 above the instrument cluster screwed into the top part of the dash), pull the dials and stuff off the light and wiper switches then undo the plastic nuts behind them. dash front should come away now, so unclip the plugs going to lights and wipers etc..
next there should be 4 screws holding the glass in front of the instrument cluster. take that off too.
probably 4 screws holding in the cluster itself. undo these, pull it out a bit and take the 2 big plugs out of the back, then remove the cluster.
the end of the speedo cable is behind where the speedo sits. just unclip it and you should have enough access to replace it from there.
hope that was helpful. i probably made it sound complicated, but it's really simple.
obviously putting it all back together is the reverse of pulling it apart.
i'll have a look at the other end of the cable when the sun comes up if i get time, as my speedo isn't working either atm, and i'll check to see if an ae82 cable is a straight swap.
good luck.