Choose either of the two race car classes the main menu, then select "Control" and start a test run on any track. At the debriefing menu, enter ISI_BABYFACTORY as a case-sensitive code in the dialog box to the bottom left of the menu. Select "Send" then exit the race. You will now have most cars for the current class unlocked with extra money.
Note: This procedure requires editing a game file; creating a backup copy of the file before proceeding is recommended. With a text editor, edit the "SRGrandPrix05.cch" file in the "\UserData\" directory in the game folder. Scroll down to the "[CAREER]" heading and edit the values of the "Experience=" and "Money=" lines as desired.