I'm not sure if the signal amp will help much; the signal coming in is hampered by the window antenna, the antenna elements likely are only a few cm from the rear defogger wires. In a big metropolitan area, signals may well overload the head unit's tuner section. FM is tricky to get right, and the factory antenna may well be inadequate for your particular neighborhood. Drilled antennas work the best, in my opinion. Perhaps you can put the cabrio's antenna in the same location it is on the cabrio? I can understand if you are reluctant to make a new hole in a 98 CLK, though.