typical rev counter issue is the rear lights glow slightly when car and engine is switched off.
you sure you've not just got a blown bulb or bad connection.
also can you be more specific with the back light not working, do you mean the tail lights on the car, or one of the back lighting for the instruments on the dash?
gotta give clear details and we can help further.
no, not any rev counter will fit, you will need one from same veriant and spec of car.
I would think the signal will be different for the 6 cylinders so you would want one from another V6 and from the same veriant ie Euro 2 or Euro 3, which is either injector shaped warning light or engine sahped warning light respecitvely. I would think yours is euro 2, then you would want one that is same colour, ie. black and orange for sport back or silver for others.
1999 156 1.8TS Sport Pack 3
2001 166 3.0 V6 24V Super
2003 156 GTA 3.2 V6 24V
2005 156 1.9 JTDM-Jet Sport
2008 159 1.9 JTDM Lusso
1997 GTV 2.0TS Lusso
2001 147 1.6TS Lusso