GPS signals are transmitted at 1575 MHz, right-hand circular polarized. I doubt you would get any useful signal from a Wi-Fi antenna. Most GPS receivers support/require active antennas to put the preamp as close to the element as possible. Typicallly 5 vdc at 20-50 ma from the external antenna connector is available for the antenna/preamp combination.

If under your hood means your convertable top then yes you will get a useable signal. Metal roofs and engine hoods will block GPS completely. Sometimes the parcel shelf will work but your coverage will be diminished, particularly in obstructed areas.

Often you can mix receivers and antennas from different manufacturers. Active GPS antennas are ofen available on Ebay.

Lynn