Oldsports,
I am dealing with the same issue - our Lexus 330 RX has a larger and smaller antenna and I want to connect both to the stereo (the larger one is not getting great reception). Where did you find the y-cable. I'm not finding any for the combination that I need (1 small female and 1 large female to 1 large male).

Thanks

 

You don't need combine it. The big one is main antenna that supports FM and AM. If you get bad reception, that's because you didn't wire the antenna amplifier power line. For my 2009 RX350, it's an orange line nearby two antenna plugs. There is an orange line plug. You need connect to your aftermarket radio or navigation ANT line.