Question: "Can you guarantee that you will find and fix my leak?"
Answer: No, we really can't. We wish we could, but cars are complicated things with hundreds of places where water can get in. If you buy our services, we will guarantee that we will find at least one leak, we may even find multiple leaks, we can't guarantee that we will find them all on the first try or that your car won't develop more leaks in the future. This is a diagnostic process, and sometimes it takes longer than the three or so hours which we spend on your car during our inspection. Sometimes you have to fix the leaks you know about before you can find others. It is a bit like trying to find a hole in a submarine that has sunk to the bottom of the ocean. You might be able to see the big hole in the side of the hull, but until that is patched up, and you have the sub in dry dock, you aren't going to know about the leaky hatches.
It is I'm afraid very common that when a car gets to a certain age, problems which cause one leak in one place cause leaks in other places too. For example, rubber grommets and seals begin to leak, these are not expensive to replace or refurbish, but sometimes they are not obvious or even missing. So we just have to go through a process of elimination.