Time-consuming.
If you have had any work done in the boot, or anything big and awkward in there that has maybe scraped against the sidewall, then look in those areas first.
If you have recently changed a stop/tail light bulb, make sure it is in the right way round. They should only go in one way, as they have offset bayonet lugs, but it is possible to fit them the wrong way round and this can cause all kinds of weirdness.
First thing to do is to look at the bulb holder and the connector - it may just need to be cleaned. There is a removable section of the boot lining covering the rear of the bulb holder so take that off, undo the electrical connector and see if it is clean - use contact cleaner spray if it looks dirty or corroded. Also check for loose wires in the connector. The lampholder being plastic, the earth connection is taken in as one of the wires in this connector.
The earth point for the rear end of the car is inside the lefthand (nearside) boot compartment, behind the lining. There are four plastic caps that retain the lining and they are push-fit with serrated ridges, so they are likely to break when you lever them out (there's no other way). They will need to be replaced if they break, but one of the parts dealers should be able to supply you with new ones (Sere Motors in the dealer section, or try GSF or similar). The metal boot wall is the earth connection, and there will be a threaded post welded to the wall with a wire and tag held down by a nut. Check that the nut is tight and that the wire isn't broken somewhere.