I think 86 is the last year of the old school shaft mount radios. You may have to cut the dash to mount a much later head unit.
I am not sure if any one makes plug in adapters for these. You could try the Crutchfield site and similar places.
In installing three stereos into my 79 over the years, and a bunch of others, including our 89, I find it pretty easy to just take the wires you have, and trace them with a volt meter, from the stereo location direct to the speaker terminal.
Then you can simply splice on the connector that you need for your unit.
It's also easier to do this that to run new wires, in my opinion.
I have only installed basic units, all but one without an amp, etc, an with speakers in the stock locations. Nothing fancy or with any serious power.
