There were two different antennas in the GLL models I believe. It's called diversity tuning. There is an antenna hooked into the glass at the top of the hatch, and then the regular antenna. Your best idea is to get a Y adapter and hook them both into the back of your new Headunit.
when I replaced my radio....had the same question. I could not find a Y adaptor anywhere so I just plugged in the antenna that got the best reception, which was the main one (not on the rear hatch window).
how does this diversity tuning work? does it switch between the antennas or does it use both combined. and when you hook up a new head unit to it, does that mean the unit will be using both antennas at the same time? etc etc.