How do I get to the antenna?

I bought a replacement power mast assembly (nylon cord plus mast) and I tried to feed the new mast into the antenna but it's not working. It doesn't "grab" the cord at all; I tried for about an hour and its just not working. I guess I need to get into the antenna assembly to see if there's part of the broken mast in there or??

I can't figure out how to get to it. Do I go up through the fender (take off the plastic inner fender cover), or do I get there from up under the dashboard?