I wouldn't worry about the speedo head unit.......until after I have lubed the cable and IF, that didn't fix the problem.........then I'd go on to the next thing.
To remove the cable from the top:
There is a push release connector. Push down and it releases the cable housing/sheath.
The cable is contained therein.
Sometimes the cable can be removed completely and sometimes it can't.
As for the speedo cable lube........graphite (liquid form).......I got the one from the auto parts place (ask for it). It gets squirted into the housing, it'll work it's way down. Thus, it was a good idea to start at the highest point (speedo head).
IF the cable is able to be pulled from the sheath.....why not go ahead and grease it w/high quality grease.
Reconnect it and you're done.
Aloha, Mark
PS........
Does it matter what kind of lube you use?
Yes, it does.......thus ask the parts guy for the specific lube. When I mentioned, "high quality grease".......remember, some grease will congeal........which I don't want.
As for the head unit, "lube" or taking it apart........I'll stay away from a DIY fix. But, it's your vehicle. So, do as you please.