Very excited about this. I love using old technology. Broke out my old (AX) which was laying around and configured it to stream music from my to my LR3 using the Aux input port. The configuration is below. Once you create a network, select it on the idevice settings wifi section. Then from your music app or internet radio app, select the named speaker and voila!!! Works beautifully. Turn up the volume on the idevice.


From an Apple Airport Forum DIscussion:
"
Yes, you can use the Base Station (AX) without an Internet connection for http://itunes.apple.com/ streaming. The AX would not get a Public IP address, but it would have a Private IP address of 10.0.1.1 ... and it would be providing wireless clients connected to it with Private IP addresses in the 10.0.1.2 - 10.0.1.200 range.

Note: The following instructions are intended for the 5.x version of the AirPort Utility.

AirPort Utility > Select the AX > Manual Setup > AirPort > Wireless
Wireless Mode: Create a wireless network
Wireless Network Name: <enter>
Radio Mode: <leave>
Channel: Automatic
Wireless Security: <select>

Internet > Internet Connection
Connect Using: Ethernet
Connection Sharing: Distribute a range of IP addresses

Internet > TCP/IP
Configure IPv4: Manually
IP Address: 10.0.1.1
Subnet Mask: 255.255.255.0
Router Address: 10.0.1.1
DNS Server(s): <leave>
Domain Name: <leave>

Internet > DHCP
DHCP Beginning Address: <leave>
DHCP Ending Address: <leave>

Music
Enable AirPlay (checked>
iTunes Speaker Name: <leave>

Your AX may report the following errors: 1) Configuration of WAN, 2) SNMP over WAN, and 3) Double NAT. These can all be safely ignored to stop the AX's status light from flashing amber in this configuration.
"



I first used d/c to Ac inverter to power the AX in the vehicle btu couldn't get rid of the hum so I thought of hardwiring it using d/c current in the vehile which would require rewiring the power supply to the AX. Apparently, there was a power failure issue with airport express and some chaps figured out how to rewire it with 5v power. So I took it apart, starting with a hacksaw, and did the rewiring and voila, it now works from a USB cigarette lighter adapter . Do an internet search for power supply repair and you'll find a few methods. I tried the one using the voltage regulator and capacitors but this did not work for me. I then used the USB compact fash card reader method to supply the power and this works. No hum. Next I need to add a line level input signal adjuster to get the volume up a bit and should be good. One nice thing with this is the sound is muted when a phone call comes in and music restarts once the call is ended. Also it takes several seconds for the AX to restart once powered up and you have to select it from the airplay icon on your idevice. But my phone recognises the network automatically when I'm in the vehicle.

In the end, I have this airport express, to which is taped a compact flash card reader, but was able to tuck it behind the center console (rt side) and run the stereo wire to the AUX input, hidden, and spent many hours getting this to work when I could have purchased a $15-30 blueooth adapter. But this method used parts I had laying around and it was a fun project.

 

Nice! I've got an old AX lying around, too. Any pics from your install?

 

I can snap and post some pics this weekend.