I have a similar issue. I have done quite a bit of research on it. Basically, the reason that it works when you are in park is because of a short. The wires that run from the button, go over the steering column. After years of shifting with the handle, the wires (or wire) rub raw. When you are in drive the wires lay flat because they are extended, and they short out. When you are in park, there is enough slack in the wires to keep them off of anything. That is why it works at that point in time.

At first mine was just iffy. It would sometimes work and sometimes not work. I bet if I try pressing it while in park I will have the same thing going on. Basically you need to get in there, find the short and fix it. I need to do this same thing. I will try to find the link that I was reading on what to do.

Hope this helps