Couple 3 questions? 1988 2wd Short bed

Hi All

First Post, thanks for checking it out.

I have a 1988 2wd shortbed and need to investigate and fix a few items
that dont turn up in the Chilton/Hayens ?

The Speedomiter does not work. Oil/gas and dome light work but the MPH
arm does not. Any ideas on how to investiage this?

Also

My horn does not work? what is the best way to trouble shoot this?

And

And I have a used toyota celica cassete stereo that I want to replace
the busted stock one in the uper portino of a double DIN. Is it easy?
should I just dive in and start unscrewing things?

the install DR. has the instructions for an 89-96, do they work for
the 88?

Thanks to all for any help

Adam