Speedo not working

[math]

Hello everyone, well my speedo stopped working.

The needle started bouncing a bit and then just quit.

I've replaced the speedo cable and still no luck

I also connected the cable and disconnected it from the back of the speedo to see if the wire inside of the sheath was spinning when i drive and nope, not spinning.

Took out the cable and tested with my hand that when i turn one end the other end turns as well, so the cables aren't broken within the sheath. I think it's inside of the transmission. I would love a helping hand on where to go from here please :'( I've never opened up a transmission before and it's scary haha.

Oh, i wanted to note that the transmission/gearbox is a manual 4 speed from a w123.