a) unscrew the housing of the instrument - 1 screw at the top,
b) unscrew the insturument itself - 2 screws
c) take the cable connector off the instrument
d) replace the light buld that lights up the odometer.
Carefull before you unscrew the whole lot.
Please first check you manual. There is an option to the lightning of the head unit.
It is possible to light up only the left/right side. So check this option first.
But if it's ONLY the tripometer, see message above....I guess
 I just noticed you are talking about a 156. Sorry.[/edit]