Gear oil is being forced up the speedometer cable?

I had my radiator, alternator and starter replaced to keep me from being stuck in the snow. Last week, I smelled a strong smell in my car, and I found some kind of oil leaking from somewhere behind the dashboard down to my brake pedal. I took the car to garage, and they said the oil is from gearbox via speedometer cable. They replaced the O-ring and seal and charged me $85.00. Now here comes the worst: It starts leaking again! Before I go back to the garage, I am wondering if you could help me out by answering the following questions at your earliest convenience? This car is the only one my wife and I are using for everyday living, so please. 1. Is the oil really from gearbox, have the garage guys really made the correct judgment and done the right thing? 2. According to your experiences, what will be the next thing that will go down most likely or what things I should specially keep an eye on as for preventive maintenance?