By far the BEST way to listen to music off an external device is this cable.

It lets you charge the ipod while using it and control the song selection with the ipod click wheel instead of the headunit.

 

Bringing this thread back to note from some research... edit: turns out multiple threads on different forums came up with the same solution, like this one that I looked up this evening:
http://tlcfaq.com/main/2012/05/land-...stereo-wiring/

However the 4Runner thread listed in Reyno's original post also covers the same material with specific part numbers if you dive into it.

Oh well... since I've already made this post I'll just recap for the benefit of anyone wishing to get the part numbers in one thread:

The smaller of the two antenna connectors can be adapted with a Scosche MDA-B Micro/Delco Antenna Adapter Tip and then that and the standard Motorola antenna plug can be inserted into a Metra 40-UV43 Motorola Antenna Y-Adapter that goes right into your aftermarket stereo.

As Reyno stated when he made this thread years ago there will be no diversity antenna function but this will combine both antenna signals.

Add to it Raine's SC300/400 aftermarket stereo thread where he shows how to make your antenna go up and down manually with an aftermarket stereo (and an interrupt switch if you so desire) and this would allow basic FM reception from the rear window glass antenna and additional reception from the large aerial whenever you want it via a switch.

 

I thought the diversity antenna refers to the two source signals being combined into one? Does it actually mean there's some sort of internal switching in the stock head unit to select the stronger source? I would think the raised mast is always going to have stronger reception than the window grid, meaning the diversity is only a fallback for when the mast isn't working?

 

Yes, the OEM head units are supposed to be able to dynamically switch between the rear window glass antenna and the mast antenna in order to use whichever gives the strongest signal for a given radio station.

Additionally the stock head unit is able to slightly vary the height of the mast antenna depending on the radio station that is being selected... and maybe signal strength as well but I’m not sure on that last one.

Aftermarket head units will just take in whichever antenna inputs you give them, so combining the standard Motorola plug and smaller plug can improve reception of all signals but there will be no dynamic switching between either one and no varying of the aerial antenna mast height.