Hi grib, 
As far as the reporting agencies are concerned, you don't need to
alert them or tell them anything.
When lenders pull your scores, the inquiries will be coded as auto
loan inquiries. This is how Fair Isaac (FICO) and the credit bureaus
will "know" what you're doing.
As long as you do all of your rate shopping within 14 days, only 1 hard
inquiry will be counted against you. The FICO scoring software assumes that
you are planning to buy just one car, so don't worry about multiple dings
on your reports or losing excessive amounts of points.
The same thing would hold true if you were applying for a mortgage.
When mortgage lenders pull your scores, they code your inquiry as a
mortgage. Once again, you can shop around and apply with different
lenders within a 14-day window but only be charged 1 hard inquiry.
So to review, how the lenders code each inquiry will tell the credit bureaus
what you're doing. No need for you to do anything....
Hope this helps. Enjoy the evening!!!! 
CanDo
"The right attitude is everything"