I'm trying to install aftermarket radio on my 1980 4 door, but can't find Anywhere how to..
Can someone please help?

Sure!

The first thing is, you'll need to let us know what brand the radio is. We can usually find the color schematic based on the brand and model.

I'd strongly recommend against thinking you're going to integrate a new head unit with the ancient wiring - If you're lucky, one speaker might work.

Since you're probably going to want to replace speakers too (and you should, since a 1980 210 had cheap paper cone speakers that are likely dust by now), the smart move is to run new speaker wiring from the head unit to the F & R doors.

Thank you for responding to my post.
I have 2 headunits and don't really care which to put in as long as it works. Below is the picture of them. One is Sony and the other is Clarion or something like that.

http://postimg.org/image/scr7fzef3/

The original radio that's on the car now is SPSaudiovox. Pictures is below.

http://postimg.org/image/giy5qbi67/

I'm also going to get the speakers for sure.

Thank you !