I would recommend checking out some of the other transmitters before going for the one at Jula. A lot of the transmitters only transmit 1-2 meters, if they don’t mention the range in the specifications the chances are it is limited. If you look at the one from Claus Ohlsen, same price, you will find it has a much further range (up to 5 meters). I have tested mine in a number of cars, aerial front and back, and had no problems.
Also be careful of the frequency range. The one from Jula is preprogrammed with only a limited range and may not work well on all radios, the others, like the Claus Ohlsen one, have the full frequency range and the possibility to program different settings so you can tune it to different cars and your home stereo.