How To Remove the OEM 'Concert' H/U
Fab up something that looks like this.
Believe me, this is just scrap sheet metal, about 1/4" wide, and cut/sanded to this approx. 45 degree angle.
(Yes, I used a belt sander!)
Stick it in the slot like this, with the beveled edge to the inside of the unit, like is pictured here.
You will need to stick it in far enuf to compress this spring. You will feel a certain amount of resistance.
Your tool from the inside, will draw this spring in.
You can then wiggle the unit out of the DIN hole.
Remove the tool(s), and push the unit all the way in. You may hear a small 'click' when it re-engages.