[Too Tall]

I have an OEM Am/FM cassette that I took out of an 88 suburban.
It worked fantastic in the Burb before I took it out.
I had a friend who's very good at electronics have a look to see if he could add an auxiliary plug and delete the cassette. He didn't pull anything apart. Just tested for power supplies to the different components.
Now with it all installed and hooked up, all I get is a slight buzz/hiss sound out of the speakers at full volume. On radio or cassette.
My friend checked to see if he'd done something, but nothing seems to be amiss!
Any thoughts?!
Thanks