Actually, it's that the receiver is not that sensitive. Typically it takes very little standing between a GPS receiver and the satellites to prevent reception. I can't imagine an antenna buried inside the dash, inside a car would be very effective. In fact I have external antennas mounted for my GPS units and the improvement of reception is quite significant. That may be what's needed.

Alternatively "repeaters" are sold that relay the signal received from an external antenna to your internal one for cases where a direct wire connection is not possible.