So I've learned a bit since this last past.
Many (all?) Toyota and Lexus radios have two things you need to keep in mind.
1. There's an amplifier on the antenna already. This amplifier needs power, and there's a lead in the antenna harness that needs to be connected. This goes to the "remote control" blue/white lead coming off the head unit. This will add 12V, which will power the amplifier, which will improve your reception.
2. Toyota and Lexus radios come with 'diversity antennas'. This means the car actually has two separate antennas, and the stock radio is smart enough to use the second antenna to cancel out interference from the first one. On the Toyota antenna harness there are TWO antenna cables. One has a standard sized antenna connector and one has a smaller version that looks similar. Only the bigger one will fit into the new radio.
As most aftermarket head units do not support diversity antennas, it is impossible to use this functionality. The impact of this limitation is that reception in downtown areas is poor, since there's a lot of interference from signals bouncing off all the buildings. This problem is impossible to overcome without replacing the vehicle antenna.
I have discovered that you can combine the two antennas using these parts.
1. Scosche Mda-B Micro/Delco Antenna Adapter Tip - converts the smaller antenna jack to a standard size
2. METRA Ltd 40-UV43 Motorola Antenna Y-adapter - combines the two antennas into one.
Using these did improve my downtown reception, but it is still not as good as I would expect and there are still long stretches of road where I can not receive reception where I previously could, but - for the price it was worth the improvement.
_Am