First off the speedo cable on an RT can be changed without any dismantling of the dash at all. Mine fell out of the bottom of the speedo a while back and the speedo stopped working (of course). I just turned the handle bars out of the way, screwed it back in and tightened it up. From there the cable just drops down to the front wheel hub where one phillips screw releases it. No fairing disassembly required at all. I usually pull the inside of the cable out once a year, wash it in WD40 or gas and then grease it up in my hand before re-inserting it from the bottom up.
Secondly, squealing and erratic behaviour is sometimes caused by lack of lubrication. Fixing this problem does require you to remove the black plastic dashboard, unplug the wiring harnesses underneath it, remove the speedo cable, trip meter shaft, the lighting connectors and the nut holding the speedo in place.
Once out get the assembly flipped over such that the cable hole faces straight up. Then very carefully drip
ONE DROP of sewing machine oil on the outside edge of the square input shaft that spins. Use a very small screwdriver to spin the speedo cable input around so that the oil gets in there and works it's way in.
Do not use more than a drop!

You run the risk of it dripping right through and onto the inside of the glass face of your speedo. But this tiny amount of lubrication often cures what ails the speedo. This problem happens on GS bikes more frequently because things are more exposed.
A wise man told me doing this once a year on his GS has prevented the input shaft from getting sticky in the first place and he has fixed a bunch of them doing basically what I described.