Just be careful with these. My wife had an iPod model of these, and a few years ago, it fried my iPod just from cranking up the car. That one hooked up through the data jack on the iPod (not the headphone jack), but could still send an unwanted surge to your phone depending on how its wired. So, I suggest if you do get one of these, unplug it BEFORE you turn off and turn on your car, if just to be safe.
Regardless, I've found Monster FM transmitters to be so-so, at best. Most brands on the market today are so-so, at best, as well, so its definitely not the best option. I have an old Kennsington model for my iPod and its AWESOME...still works after a good 8 years of ownership and gives a radio-quality signal (no static, barely any quality degradation). But, the new 'Liquid FM' models from the company are horrid, so avoid those.
Your best bet is:
1- Get a new stereo with an auxiliary jack on it or with BT capability.
2- See if you can retro-fit your stereo with a jack (easier on some cars, a lot harder/near impossible on others).
3- Buy a new car with a jack