Ok, try this:

Go to Walmart of Autozone and get a set of "radio keys" and yank the head out of the dash.

At the back of the AM/FM/CD/NAV head will be two black antenna cables, unplug the SMALL (GPS) one. Slide head back into dash and try THAT.

If the L.O. of the NAV unit is radiating, it' would most likely radiate thru the cable/antenna of the GPS in the NAV.

If that does not do it, well, I tried.