IMO, the only dumb people are those who think that everyone is born with this information already in their head.
To add to my previous post, "The electrical connectors go to the computer" assumed that you did NOT mean either of the 2 large, multiconductor electrical cables which are (1) the transmission shift lever position switch cable or (2) the transmission electronic control cable.
The 'firewall' is the (usually) vertical metal panel which separates the engine compartment from the passenger compartment. It is a little less well defined these days with the compaction of car bodies such that the firewall and the 'transmission tunnel' have melded into each other by design.
The speedometer 'cable' is a mechanical cable that does not use electricity. The faster the car goes, the faster the center of the cable spins and the more the needle moves in the speedometer gauge in the dashboard panel.
This cable plugs into the transmission very far forward in the transmission and in fact is plugged into the 'front differential' portion of the transmission. It connects vertically from above and is held in place with a spring clip. No transducers are involved unless you have an oddball conversion of some kind or a car made for another country, AFAIK. There is always something I haven't seen.
This cable is hard to see but is near the front 'differential' oil level checking tube/stick (NOT the same as the 'transmission' oil level checking tube/stick) and is on the right side (passenger side on American cars). Since the cable connects directly from above, it will be the upper most connector on the transmission, except for air vent hoses.
Climb in the engine compartment, get your head as close to the firewall as possible, look straight down on the right side of the transmission (passenger side), and you should see both the differential oil filler check tube/stick and the speedometer cable connection.
You symptoms sound exactly like a failing speedomter (mechanical) cable since usually the square end of the cable rounds off with wear and begins slipping.
Hope this helps.
Let me know how this turns out or if you have other questions,
Ol' Rep (not Subaru)