it depends on your model....... k jetronic then it could be a box thing on top of the gearbox which also does the mfa........
if you have a digifant...... then it sould have a 2 wire sender which as far as i know just screws in.......
if you take the bulbs out as i ddint on my old mk2 i use to have then it wont fail the mot....... reverse lights arnt required!

i thin the senders arnt much so worth replacing......