Have a look at Dave Barton's website where there is very useful stuff about speedos and fixing their problems.
https://www.prancingmoose.com/740-odometer-repair.html
The speedo will not move when you turn the car on only once moving.
The plug behind the radio head unit is probably the fly wire for the original amplifier which was fitted to these cars as the original units didn't have a decent power output. It is not necessary with modern units as long as special by-pass wiring is fitted to link the speakers up at the old amplifier.
Bob
