You can try fixing them, there is a really good guide as previously mentioned. Unless you replace the brushes then chances are the repair will only last a short while. Clean out all of the gunk and replace the brushes check your old brushes and chances are they will be down to the stubb. Replace with new ones and they should give you plenty of life. The brushes cost less than a tenner, thus saving you a few quid, the only cost to you will be a couple of hours in your garage removing the door cards and the motors, cleaning them up and fitting new brushes and putting it all back together again.