While not exactly a cheat, this will allow you to race some of the wanderers that you may have missed in order to unlock the final racer.

Rival 316 Will appear when days are a multple of 7, you must have a shindowa plate

#319 you must have an import car and 180 days must have passed

#326 FF type car needed, days must be a multiple of 10

#328 6 cars needed in garage

#329 will appears between 9-11pm

#330 shonan plate needed, days must be a multiple of 7

#331 will appear between 2-4am

#334 must have under 30mi. on odo.

#338 first two numbers in days total must equal four (ex. 220days) and you must be using a two seater

#340 odo. must have 2000+ mi.

#341 must have 25 cars in garage

#342 odo. must have 120mi. or less and you need to use the stock exhast.