I did my own LED install as well, and I have the same problem. The reason why you have a dark spot is becuase of 2 possible reasons:
1) The LED is not touching the contacts to the radio circuit board
2) U burned the contact
Only way to find out is to open it and check. I had to do that a couple of times. And since it was my FIRST time doing Sauldering, I ended up burning the contacts. I'm glad I did I better job on the gas gauge. But not a bad job.