I had much of what you describe on an 86 K75, 67k miles. Turned out it was several different things, some that were really easy to fix.
GPI very dim and no back light: I've got signal so it's not the switch. Learning to live with that one...
The odometer/trip log didn't advance, Speedo works: Removed the speedo assembly, replaced the "soft gears" with parts I got from Palo Alto.. $78 all up. Got here in a couple days...which is good for Hawaii. Happily counting miles now.
Speedo stopped working shortly after I had it back together. Took it apart again and found a spring... exactly like the one on the odometer reset knob... stuck between the back of faceplate and needle swing arm. The needle is happy again.
I changed the display backlighting to LED from the Duck when I had it apart. Cheap, easy.. but if they are not connected properly they will not work. Trust me, I know this..
Good hunting!