I suppose that this is the same "Qwerty86" as you?
I did not know that you made cars (or forgot it...)
http://www.3dactionplanet.com/carmageddon/cars2_Qwerty86_page1.shtml
I removed all default cars except:
- red eagle, as it was the default one. Is there a way to set another car as the default one?
- police car, as it is used in a mission. Can it be replaced?
- dump truck, for the same reason
I suppose that all others could be removed/replaced?
Where can in find the Citroën DS?
Yep, that would be me. I made a few cars for that game. I also made a few cars for Midtown Madness. That's pretty much my claim to fame. Citroen can be found here:
http://wasted.polygonized.com/downloads/index.php?act=view&id=685
To set the default cars, you just use a program called "Carstockalypse" and move whatever car you want to the top of the list to replace the Eagle. Carstockalypse will also open up (or "unpack") the game's files to allow editing.
For the Bruise Brothers level, you just open up your races.txt file (in your "data" folder) and scroll down to the level entry. You'll see an entry that says "Number of explicit opponents" and a bunch of 51s. The 51 corresponds to the opponent number in the opponent.txt file. Replace the numbers with whatever you want and you'll have whatever car you want in the level. They don't all have to be the same number either as long as the number of cars/opponents matches the initial number. Same thing for the dump truck.
I personally could not make it run fine under XP. I kept my old Celeron 366 MHz mostly for that 
