USB is a digital signal but an Aux. input jack is normally analog audio, so
"USB to audio" isn't possible.  You might want to take the Dell unit and
your car to a Radio Shack as they may be able to ascertain the correct
connectors.

Tom

If the jack in the car is just a standard stereo mini jack (3.5mm) then the
easiest thing to do would be to connected to your Dell DJ's headphone-out as
it has the same kind of jack and the output is analog.  A cable with two
male jacks (one on each end) can be found at pretty much any electronics
store.

Cheers,

Pete

All you need is a standard male to male 3.5mm stereo jack patch cord to
take the signal from the headphone socket to whatever you're connecting
to.  The level may be a bit low, meaning you may have to max out the
DJ's volume level, but it works just fine.

I have one patch cord (3') that I use for connecting my Dell DJ20 to the
[installed myself] AUX input jack in the glovebox of my 2004 330CiC,
another (3') I use to connect the DJ to the aux input on my motorcycle
audio, and one (1' long) that I carry around for any other purpose.  The
3' ones are audio accessories from Radio Shack, and the 1' one is a
computer patch cord, also from Radio Shack.

Jon

Is the AUX jack factory installed? If it is, what models have them? I added
one to my 2003 E39, the kit instructions said to put it in the glove box.
Bad idea...arm rest good idea.