There are 2 options to connect to the internet wirelessly using your carPC (or home PC).
Option 1: 802.11 WiFi, you can connect to it if you are within range and the Access point (AP) is open for you to use. There maybe some legal isssues here regarding to unauthorize connect to some one network, but that is a different thing. With this option, all you need is an 802.11 WiFi adapter (PCI, USB, PCMCIA).
Option 2: Using your cell phone carrier as data provider. If your cell phone carrier provide/has GPRS and your phone is capable of. You have to pay your cell service provider/carrier for them to turn the option on for you to use. With this option, you have to buy for the service (just like DSL), phone capability , connection between cell phone and PC (Bluetooth, data cable, ... IrDA). Technically this is broad band internet for cellular phone and called GPRS and your cell phone a broad band modem. DSL is broad band internet for home/land line phone.