Wireless FM modulators are always a dicey proposition. The FCC doesn't let them get too powerful and in a big city, you're just not going to get a good sound with one around here.
I've noticed that a big factor is actually your headunit. Some of them do well with the wireless FM mods. Some of them, not so well. You never know.
You are better off just buying a new headunit that has an AUX input, or, if you are in love with your head unit for whatever reason, you can often convert a CD changer port to an AUX input. There are lots of solutions for that. Alternatively, there are some hard wired FM modulators, that power your device and when you turn it on, it cuts off the FM antenna and feeds the device into the FM directly.
Of course, the FM wireless mods built in to XM units work purrty good.
I can hook you up with a free one.