the factory radio swithces between the 2 automatically depending on which gets better signal. the onyl way its not hooked up is if you put an aftermarket headunit in and didnt keep it plugged in (if you follwed the antenna lead maybe 2 inches back there should be a y). i just left the mast antenna unplugged so it never went up. no issues. ever.
do you not have an owners manual? i know i had a scan of the manual page for this exact reason, but i must have gotten rid of it