For starters, do you have an amplified system? My old 1.8T Sport had the rear speakers that were amplified by a crappy bose amp. Check if the fuse has gone on this?
Other thoughts: Wiring to speakers/amp OK?
To be honest, if both the rear speakers are not working, I'd be checking things that are common to both of them, which would be the connections from head unit, amplifier, fuse for the amplifier, etc...
Have they ever worked or have they stopped working recently?
AndyMac is probably the authority on this, but the above should get you started.