@terradelu: I'm assuming that, since your car doesn't have a 3.5mm aux input, it's old enough to have a cassette player. If that's the case, I'd recommend a car cassette player adapter. They don't work as well as a dedicated aux input, but the signal and audio quality are much cleaner than an FM transmitter. If your car doesn't have a cassette player, well, I can't really suggest anything. Sorry.
Show me a picture of the transmitter. If it is a certain type I can show you how to hack it to make it more powerful, assuming you are willing to get your hands dirty.
The thing is those transmitters are intentionally weakened to comply with FCC regulations. Inside of them is a small bolt that you can turn to increase the signal strength. You will need something to pry it open, a soldering iron and a fuckton of patience.
Just wanted to bump this thread to say that I recently took the whip antenna off my car and the difference in clarity from my FM transmitter is night and day. I was planning to buy a new head unit with a line-in jack (the car I got after my beloved Honda was totaled doesn't have a tape deck), but without the antenna on the car it sounds just as good as a direct connection.
Iv noted with mine i get a ringing noise like its not grounded. Iv tried removing the antenna, changing fm to many different channels. The problem with the low 80.0-90.0 is that mexican raidio is now using those in the area i work in. I found that turning my tribble down to -8 and my bass up to +3 is the only way to hide the ringing noise. Iv tried multiple fm transmitter ls, and tried them in multiple cars, all have that problem exept the battery transmitters, but i cant afford buying batteries all the time.