I made a simple FM antenna for my Sirius Satellite radio car kit's FM transmitter, and now the sound quality is excellent.
The length of the antenna is not critical. I have listed the optimum length for 88.5 MHz, but your car's external FM antenna uses one length to cover the whole FM band because there is not a large penalty for being off slightly in the length. Similarly, the antenna wires don't have to be kept perfectly straight. I tucked mine into the seam along the top of my dashboard so that it's mostly out of sight.