Most of what you need to know is on my web site under Matching.
A Hamstick, and similar, small, spirally wound, antennas are rather lossy. This pushes the input impedance up. Usually the mismatch is small. However, when you mount one close to the body of the vehicle, the stray capacitance pushes the input even higher. This is especially so if the main coil is close to the body.
Once you determine what the input is, then the fix is easy. The aforementioned article will tell you how.