The wiring harness goes to the head unit and then there is a separate harness that goes from the head unit to the tape unit.
When I installed a CD play on my 90 K1500, I got a kit with the CD player from
www.crutchfield.com. The kit included a wiring adapter (no need to cut any wires), a CD pocket (where the head unit was) and a new trim piece so that CD looks factory.
This may help:
http://www.installer.com/cars/chevrolet.html
I can post a picture if you want to see how mine turned out.
//2000CAYukon