Someone in my area is interested in buying my stock delco cassette stereo but I'm unsure if it works since my GTA already had an aftermarket stereo installed. I'm wondering if there is a way of testing it via the pins on the radio since the car didn't have the original harness anymore? Thanks in advance.

you can show him the drop test !! you need to have the harness test it...

What is a drop test? Yeah I figured I needed the harness but wasn't sure since I inserted a wire on the 'bat' pin and on the 'grd' pin, touched them to the + and - terminals but didn't power on!

Hold it up and drop it! If it bounces it passes the drop test and the bounce test

You must be bored huh