I'm trying to disconnect the speedo but I cant seem to get a good look at the back of due to a vent that comes out near the windscreen in the corner.

Can anyone give me some suggestions?

 

there is a spring-loaded prong that has to be depressed to remove the cable from the speedo housing

 

And or go into the engine bay. Find the speedo cable. Shove the excess slack INTO the crew compartment. That should shove the combination meter towards the drivers seat direction and give you a touch more slack to depress the release lever on the back of the cable and disconnect it. Oh and remove the steering wheel to give you more room to work if you have not already.