In my Sprinter (and the E-350 before) I can usually get at least a weak signal, but usable. But as soon as I put up the silver reflective thermal sunscreen thingies, the signal goes poof. Sometimes I can leave one of the windows uncovered (use a curtain instead of the reflector) and can still get a usable, weak one-bar signal. If the signal is a very strong one to begin with, the reflective thingies (it's an industry term) doesn't prevent me from getting a signal.
Most laptops don't have a connection for an external antenna, and most laptop WiFi's are inherently weak, which is why you can get a signal inside the J but not outside. A PCMCIA cardbus WiFi adapter, or a USB WiFI adapter, will have many times the power of the built-in adapter on the laptop, which will result in getting a much stronger signal, especially if the adapter has a movable antenna that you can raise and lower (and break off easily).
Many cardbus adapters also have a connection for an external antenna, like a magnet mount for outside the vehicle. An external antenna like that is about $20 or $30, and a PCMCIA cardbus adapter will range from
$40 to $120, depending.
The truckstops have the card adapters, many have the external antennas, as well. Couple of other options are:
Buffalo Technology Airstation
http://about.pricegrabber.com/search_getprod.php/masterid=10011833"
which is what I have, and works great, or the:
Hawking Tech. Hi-Gain Wireless-G Laptop Card
http://about.pricegrabber.com/search_getprod.php?masterid=6974762&search=hwc54d
which I know also works well. My problem with the Hawking card is that I'd break the flip-up antenna within a week. But that's just me.