I used to have this condition few years back then, and no matter how i kill, it just grows.

Until one day i buy RM4 cockcroach killer spray can from MRDIY, and wait until midnight (usually they will party after night time around 10-5am)
I spray 1/3 can of it aimming at the floor mat and the spots they usually hidden and wait until next day morning

boooooomb, all dead.
And i cleaned up everything inside the cabin, the leather, dettol wash etc to make it perfect, it was cockcoachless until today.