New better and much quicker method now available here!:\r
Only works on Chapters 1-4 (new game plus) Tanners yellow Dodge Challenger will give chase and you can drive it as a cop car, follow these steps accurately and itll work! :) Please watch the full video as it will tell you when the glitch stops working etc. What basically happens is that Tanners car temporarily replaces the Corvette cops cars in free roam.\r
Glitch found by the69bandicoot.