One method I have used on removing insulation/tar/bitumen etc,
from the inside of the car is to purchase a couple of gallons
of turpentine, 3M "steel" wool (actually a very tough plastic,
plastic spatulas, and have lots of news papers handy.
1 - plug up any holes in the floor-pan where the turps might
leak out. (obviously remove all seats/carpets etc etc)
2 - pout 2 gallons of turps on the floor-wells
3 - let it soak for a couple of days
4 - start wiping the floor with the 3M wool and it should
just come off
5 - clean up with newspaper.
this has worked for me when preping all my race-cars where
you have to remove all flammable components from the driving
compartment. After this procedure the floors were left absolutely
clean and after washing with very soapy water, ready from primer/paint
Obviously you are dealing with flammable liquids here, and quite iterating
as well - so use you judgement.
Alternatively wait for winter - leave the car outside in sub-zero temps and
the insulation/tar should just chip off