1981 Westy 122K    speedometer acting weird.  It stayed a zero up until maybe 30  then started to work.    Then when I slowed it dropped in steps, when fully stopped it read 20  mph.    Later worked OK.   Pulled spare tire and found the cable from left front wheel goes to a mid-point junction box behind the front bumper and above the spare tire. Then a second cable goes on apparently to the speedometer.     
The first cable appears not to original as it is way to long and some one looped it in an extra 20 inch diameter loop.  I added some support wires to try and make all the curves smooth as one curve seemed very tight.  Hope this works.
 Any way, looking up the replacement cable part, there is one described as bye passing the junction box.  What is the function box for and what is the best plan for keeping this 1981 going?

 Glen