1998 Vauxhall Corsa rear lights

Electrical problem
1998 Vauxhall Corsa Two Wheel Drive Manual 67000 miles

the brake and reverse lights worked until the icv was clean due to a build up of grease had stoppped it workin properly it was after this that they stopped apart from minor maintaince for mot no other work was done all the usual stuff as been checked bulbs & fuses the pedal switch was also checked used a circuit tester to make sure there was power to the fuse someone said an earth might be lose but as there are so many brown earths I have no idea where it can be any help would be much welcome as mot is on monday