I unlocked the 'Back of the Pack' mod through buying the Premium mod pack for 300,000 credits. The mod is classed as Super Rare in the game.
After failing to come 1st a few times, I combined it with a single-use mod I received in the same pack which gave +3 grid position, so I started in 21st which made winning a little easier. Won first time and the achievement unlocked at the end of the race.
Note from central perk 08
: "you can do it in free play with collision mode set to always off".
Also, similar mods will NOT work for this achievement- it has to be the mod called 'Back of the Pack'.