Tweak the talent files.
Consistency=0.20
Crash=2.4 //% of times driver crashed
Recovery=80
The lower the value for Consistency the more variable that driver's performance will be (thus more likely to make a mistake); lower Recovery value means less likely to correct a mistake.
//AI skill mistake variables
MinRacingSkill = 0.98
Composure = 0.30
Lower skill means more mistakes; composure is controversial, some say it does nothing in GTR2, some say it controls how "stable" the driver is - a lower value making him more prone to mistakes under pressure (I've seen no definitive effects, but it may be subtle and depend on the track and mod).
One thing that makes much of this experimental is there is a certain random factor to all AI settings that is hardcoded and can change from session to session, so it takes a lot of trial and error to make sure what you change and what you notice on track are consistent (every time you leave a track and return those random factors are reset; if you save a race all settings are also saved so further changes will not be recognized).