[Palf70Step]

From what you are describing, it sounds like your instrument cluster or speedo is bad . I have heard issues with them, but I don't remember if it is just replace the speedo or the entire cluster as a fix.

Normally, any aftermarket radios will mount in the center of the dash (where tape deck mounts), not in the original location. They make bezel kits as the aftermarket deck will protrude/stick-out as the dash is very shallow at that point Then just disconnect the speaker & antenna wire from the original and leave everything else alone for the climate controls. At least that is what I did on my 92.