Maybe you could do something hackish like attach a piece of wire to the antenna then feed it inside the car - so that you effective have an internal antenna.

Of course it would be even neater if you did that to the antenna connection behind the car stereo. If your car has a removable stereo it should be possible.

I know under the right conditions even the cheap iPod transmitter I bought off eBay can easily drown out local stations so it shouldn't even be a problem if you drift in/out of range of REAL stations using the same frequency. Its all down to how well the stereo is picking up your transmitter vs the real one. Obviously if its struggling to begin with then the real station is going to easily be stronger.

If you really want it badly enough you could completely disconnect the external antenna in favour of rigging one internally in the car but then you might lose reception of real radio stations, as you would have the reverse problem.