1) Double-click on your listbox in design mode and at the bottom where you have connected the list to a SQL database, you can choose what field you want to be the "Value" and what field you want to be the "Display Name". When you call selectSingleNode("...").text it will return the value of the field that you chose for "Value", not "Display Name". "Display Name" is simply what the user sees when they drop down the menu. Make sure that the "Display Name" is textual, but the MAIN thing that you need to do is make the "Value" field correspond with you "id" from your database.
2) After you have done this, things will be a lot easier and you can query your database from within your code to gather all the information you could want. Your query statement would look like this:
"SELECT * FROM _table_ WHERE id = \"" + parseInt(thisXDocument.DOM.selectSingleNode("my:myFields/my:Car").text) + "\""Of course, your code will be different depending on what language you're using (not exactly sure on yours), but you should get the gist of it.
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Matt Faus
Matt Faus / Microsoft InfoPath MVPQdabra® Software / Streamline data gathering to turn process into knowledge