saand,
yes you are correct, for a correct impedance match you need to have the receiver, the antenna, and transmission cable all the same impedance. your best bet is to use the cable that was used to connect the existing antenna. I'm not sure off hand what the standard antenna impedance is used for car antennas (my gut tells me its probably 75 ohms due to cost). I know TVs are 300 or 75 Ohms. For radio communications equipment its most often 50 ohms.
In general an antennas impedance is determined by its design. A dipole is about 73 ohms. A monopole is roughly about 30 ohms. A loop antenna is usually around 300. In a lot of cases they aren't a perfect match and have some tuning circuit to match them.
some good reference material
Monopole antenna - Wikipedia, the free encyclopedia
Why Fifty Ohms - Microwave Encyclopedia - Microwaves101.com
Dipole antenna - Wikipedia, the free encyclopedia