Courts have routinely taken the position that "an employment contract cannot be used to deny a worker from his livelihood in the ordinary course of business".
The NC which are enforceable are generally very specific and usually cover things like trade secrets or proprietary data (ie. secret formulas, customer list, etc).
So you leaving your current job to work at another mechanic shop is entirely unenforceable, whereas you downloading the entire customer DB and shopping yourself around to other shops (with data in-hand) probably is enforceable.