If the cable for the fuel door is like the rear trunk, which I assume it is, the job wasn’t too bad. I simply “fished” the new cable in the hole a the B pillar door jam (first taking out the phillips screw that holds in the lever). With the trunk release the cable moved fairly easily through the conduit and came out where I could reattach it. You would have to pull back the carpet and side card to access the fuel release. It should be a similar process of pulling in the new cable and then reattaching and securing it.
I was able to get a suitable cable from a bike shop (brake style cable) for about $4.00 that worked with just the right amount of cable for the trunk latch.
With the fuel door, I too am having issues with the button on the dash not releasing but my first priority with the fuel door opening is to replace the rubber stops that cause the door to pop open when the latch is pulled. Those rubber stops have compressed so even if the latch is opened the door doesn’t pop open and I have to pull it open by hand. The spring is wound and I know will pull the door open as well but it looks like the later 780 doors (90 and 91s for sure) had a different door opening/stopper design. Anyone tried to replace the stoppers? Let me know how the cable repair goes.