ah.. u only have a few cockroaches.. i experienced before small
n big altogether 10+ in a car. Bery old car my bf bought only for
$3000 ready to be scrapped within a few months.
Bought already so no choice must use.
One day we parked the car near a drain and the next day when we
open door, all the cockroaches run to hide. One glance i can
already see more than 10 cockroaches in the car running about..
URGH!! Make mi sick.. =_=""
So we got ready 2 bottle pesticides, 1 room fragrance bottle, lots
of tissues ,a small brush/broom & 2 torchlights.
In the middle of the night when the cockroaches all came out, we
open door and sprayed inside and close the door.
When close, we can see smoke inside the car! 
Then those cockroaches flying & dying everywhere...
Smoke gone, we open door and take tissue to grab out and use brush
to brush out those corpse. Those very determined one, we on torch
light and search and spray at corners, hard to reach areas.. and
caught the rest. Like that, 2 bottles used finished. 
Then we spray room fragrance one.. also finish 1
bottle. 
Next day the inside of the car smells weird and.. oily..
We can join pest control company liaox. o_O
