Let's start things off by saying the BMW M1 Procar Rally is part of the Group B (RWD) division.
In my opinion the BMW slides out a bit more than the Audi Sport Quattro that you need for the achievement below:
With that being said, you may need a bit of seat time in this car. (I can not confirm if a win is needed, however the description makes it sound like any placement will work.)
You can complete this achievement in Career, Historic Rally: Back to the 80s or in Custom Championships.
The quick and easy way would be to create a custom championship as it will only be one race. Once you click "Create Championship" it will ask for a destination. This is personal preference as I do not know what your favorite stage is, however I tend to choose Spain.
Once you choose Spain, it will ask for suface settings and vehicle class. Here you must change the class to Group B (RWD). Once you confirm it will shoot you to stage select/confirm. I personally switched the stage to the far right, a race called "Subida por carretera". It is 100% tarmac and only takes about a 2 min 20 seconds. To make things easier make sure your conditions are set to Daytime/Clear/Dry Surface and equip Medium/Hard tires.
All that is left now is to race and win the championship.
1. Freeplay/Custom Championship
2. Add Event
3. Spain (Or your favorite)
4. Class: Group B (RWD)
5. Change stage to Subida por carretera (Daytime/Clear)
6. Start Championship
7. The BMW is the 3rd car from the left
The race is about 2 min, 20 seconds