Location Of Nav System Gps Antenne For 2004 Es 330

[Norris]

I have a 2004 ES 330 with the NAV system. I will soon have a Lexus XM receiver installed and I was wondering where the GPS Antenne for the NAV system is located. I would like to have the XM receiver antenne positioned in the upper center of the trunk lid but I was told that location will interfer with the NAV GPS reception. Is that true??? Is the GPS antenne at the Bottom of the rear back glass or?? Thanks in advance for your help.