In my RV I had to do a bit of modification to get a new "Head unit" installed. I had the old school cassette from the 1980's that had the traditional dual knobs. I also had the same vintage TV and VCR.

I first had to remove the old radio and then cut out the rectangle for the new unit. I used the above mentioned sleeve as the template and in it slid.

As you can see, the radio was not all I upgraded. But the radio itself was very easy to do. I did have to hard wire in the new plug for the new radio, but that was 12v power and 4 pairs of speaker wires.