1 hour ago, Mark Maras said:

 I'm going on what I've read. It appears that the speedo has two wing nuts that are accessed from under the dash. One wingnut on the top, one on the bottom. It's been reported that they can be difficult to crack them loose. After removing the wingnuts and disconnecting the wiring, the speedo pulls out from the front. If you have a dash cap it will be difficult to get the speedo out but it's possible. Good luck.

This is an extra speedo i ordered many years ago. Should i just wiggle the plug to see if the dim brake light dissapears? Or do i need to take it out fully?

 I'd wiggle things first and check the ground. An easy method of checking grounds is make a temporary ground using a length of electrical wire with an alligator clip on each end. Ground the instrument with the temp. ground wire and see if anything changes.