Jim
as Ray says it is well worth tracing the bitmap of any pattern no matter how good it looks when you have finished in Gimp.
Now if you have neer done it
open inkscape and import your car pattern
the
Path-trace bitmap
in the dialog box you nee to play with the threshold (in this case I think I set it to 620)
then OK
you can now close that box
you are now left with one image on top of the other, click the top image and drag to the left so they are side by side.
the bitmap is usually but not always the top one, but will be the darker crisper image
(if ever in doubt of this, then with the left hand image selected go down the the colour bar at the bottom of the creen and click on red if the image turns red it's the bitmap then ctrl+z to udo.
now delete the original, reselect the bitmap and put it back on the canvas.
now lets assume you want the red/grey option
with the image selected (nothing will work if its not) go back down to your colour bar and right click on red from the pop up box select "set as stroke"
then move across to the light grey ( which is almost white in inkscape) right click and select "set as fill"
you should now have a page of light grey blotches with red outlines.
if you want to emphasise the the red outline or thicken the lines
in the bottom left hand corner of the screen you will see 2 colured rectangles these are depicting the colours you have chosen "fill and stroke"
at the right of the red stroke rectangle you will see a number the is your line thickness, of which will be defaulted to pixels, right click on the numer and from the pop up box you can now choose a thicker line, when I did the above I think it was 0.282mm and I increased that to0.75mm approx 3 times as thick.
hope thats as clear as mud for you, just give it a go and any questions just ask, theres always someone around on here that will have answer
Dave