Simple.
Make mechanics earn more xp from their services. Also add more jobs for mechanics. They only have towing which is ussles. You tow 2 cars then there aren't any cars to tow anymore.
One more thing.
Make mechanics keep their towing streak after they leave their TowTruck. Cuz your towing streak resets to 1 every time you leave your TowingTruck.
Example.: If i have a towing streak of 5 and i leave the towingtruck to get on my Bullet to assist a customer faster, i loose the towing streak of 5, it goes back to 1 when u reenter the towingtruck. So, if possible, make the combo streak of towing to be kept at the current number of cars you have towed when u leave the towingtruck.