90 f250 shift position not working
 

I have an automatic and whenever I shift the shift or positions stays in park. The little mark above the steering column doesn't move to the gear I am in. Please help.

Welcome to the site.

It sounds like the shift indicator cable has snapped.
There’s a small cable that connects to the shift column and runs up to the indicator assy. If the cable breaks the default position would be Park since the indicator has a return/tension spring.

If you know how do you think you could give me an idea on how to fix it? Greatly appreciated.

I can send you the steps for a 92 which the instrument panel differs a bit, but similar.
For a 90, you'd need to remove the R&L lower instrument panel trim, the lower plastic/nylon column half cover (underneath the ignition switch).
And, the instrument cluster upper trim panel.
Once all the trim is removed, the cluster itself needs to be removed, or, moved enough to drop the selector indicator out of the cluster bottom once the 2 screws are removed. The indicator cable loops down off of the indicator itself, and attaches to the lower column and to the shift tube via a cable loop end..
After you preview the steps, it would be a bit more clear, but if you have any questions you can message me. Its not hard job,if you have some simple tools..