AM / FM antenna

[Silver SLP]

I understand that the am/fm antenna is built into the rear window grid. I installed an aftermarket radio and now I get some FM reception, and no AM reception.

*None of the grids are broken and there is no problem with the wiring*

What I want to do is install a small aftermarket antenna, preferably on the roof near the front of the windshield. I would like to use an onstar antenna (like from a 2009 Malibu, or similar), however I understand that the plugs are different and it will not work. I will then get an antenna wire extender and run it from the new roof antenna to behind the radio.

Anyone have any ideas? I am good with this kind of stuff and I think it would be a fun project.

Thanks in advance!