Not familiar with this model, so this could be total rubbish! Assuming handbrake is via separate brake shoes/drums contained within the rear hubs (i.e. totally separate from rear disc brakes) it would seem unlikely that the shoes would wear out unless you habitually use the handbrake to stop the car. However, as steveshaw said (and I have read other similar posts) the lining material has been known to detach itself from the metal shoe.This seems to manifest itself as unusual noises from the rear hubs. Seems to occur mostly when the car has been standing as so lining material binds to drum and is wrenched from the shoe. WARNING - there are some posts where this has happened and the brake has jamed on the move causing the wheel to lock solid - obvious safety risk!! If there have been no unusual noises, the deterioration has been gradual and given the the amount you have already adjusted the cables, I would still suspect the cables. HOWEVER, would be wise to pull off the rear discs and check the condition of the handbrake shoes in any case. I would be quite happy to be proved wrong to ensure your (and your loved ones) safety. Follow steveshaw's advice first - check the condition of the shoes / drums and take it from there!