Re: 156 Rev Counter
best thing to do is take your old one out, only a few screws.
with it out you will see it can be seperated (front and back) by 3 or 4 plastic retaining lugs which can be gently prised open (only about 1mm) to allow release.
when the unit seperates you will see the back part has the circuitry on, and the front part is a sperate unit which has the rev counter in it.
There are 4 pins that send info from one section to the other, you need to pull them apart 'straight', as in don't go bemnding it side to side to seperate it.
Putting back together is simply lining up the 4 pins and pushing together.
try it on your old knackered unit first, if you break it - it didn't work anyway, that way you will see how it comes apart.
it is all very simple, and will become clear when its in your hands.
I cchanged my 5pot jtd with a 6pot V6 re3v counter, just by swapping my front with the replacement circuit board on the back.
I did not find it matched to cylinders or even petrol/diesel etc by doing it this way, although if you changed the entire unit for one with more or less cylinders you may well find that your revs are incorrect even though the unit works otherwise.