Maybe this has been covered before, but I can't seem to find anything after hours of searching. Has anyone wired up an aftermarket amp for the headrest speaker controls?

When I got my 88 vert, the door speakers work, but not the headrest speakers. It still has stock head unit (am/fm, cassette, EQ) Checked the headrest amp, and it's gone. I want to put in an aftermarket head unit, new door speakers, but also want to get the headrest speakers going. I figure finding a replacement stock amp will be pretty tough.

I'd like to utilize the controls, but not sure if I can with an aftermarket amp for the headrest speakers.