After ~60million credits spent on packs over the last year of actually trying, I finally acquired the Back of the Pack Dare Mod. For those unfamiliar with this Achievement it is :
Researching around it seems that plenty of people have been awarded this achievement when combining it with boost mods that alter Grid Position as well as no collision. I chose to simply use the latter and on the first race I used a Spectator Boost(Your car has no collision for this race). When that combo failed to award the achievement I used a Ghost Boot(Your car has no collision for the first lap of the race) since on the first race I had managed to go from 24th to 1st on lap one. I never used a +Grid Position Boost.
Both races I started with the Back of the Pack Dare Mod equipped, both times I finished first by a large margin. With it now failing to award twice there must be something going on with the achievement.
Edit: It finally awarded after many attempts.