There are only 2 possibilities, either the bulb is fubar - which you say you have changed - or the brake light circuit for the right hand cluster is faulty.
The earth part of the circuit must be ok, if i am assuming correctly and the other lights in the cluster work ok. so that leaves just the +ve supply.
The supply is fed from the check panel control unit to the cluster using red/black, so make sure there is voltage where it connects to the cluster.
If there is voltage, then the problem is on the cluster - which includes the bulb holder (or cylinder).
As Ralf has said, it may just need cleaning with emery (wet and dry) paper or sandpaper.
If there is no voltage at the cluster, then you have at least traced the problem somewhat.
If this is the case, you could temporarily run a bit of cable from the left cluster brake light to the right, BUT the light will remain on, as that part of the circuit is still faulty.