i dont think its a good idea, first there are a lot of metal enclosures second a lot of rf noise from spark plugs etc, maybe its best if you use the ext antenna cable but instead of using that antenna you use a patch on the outside, you can get about 3db gain, by personal experience you will get good snr from about 6 to 7 satellites depending on when and where you are.
anyway you can always try. By the way attached are lab report of one gps antenna i made long ago( its in spanish but it may not be hard for an automated translator to do the job.)