also quite interestingly, the white airport extremes use a stock Apple 802.11g airport card. Plugged right into the motherboard in the base, and the antenna uses a standard connector to the built-in antennas. So it shouldn't be too difficult to wire up your own external antenna. Just be sure to keep the antenna leads short. Getting it up in the air better and away from large areas of metal is always a good idea. (no, don't sit it on top of the fridge either)

They use an MMCX connector. Here's one with a bit longer coax than I'd care to see. (good though if your radio is in a hole and you need to go a ways to get up to where the antenna needs to be) http://www.l-com.com/wireless-antenna-24-ghz-5-dbi-desktop-omni-antenna-4ft-mmcx-connector

search eBay for: mmcx antenna
make sure it's tuned for 2.4ghz
compare DB gains, bigger is better. consider each foot of coax to eat 0.5 DB of that gain. (that will mean NEGATIVE gain in some cases. but getting your antenna outside your car instead of inside it can be worth it, where simply BEING in your car may be giving you a 10db loss)

I work for the Department of Redundancy Department