I managed to connect a small speaker to the TX2 car remote chip SO pin 8 and now I can hear the different PWM pulse signals the buttons forward, reverse, left and right generate. Pretty cool! 
The signal has rougly 1.3 volts when I measure it with a multimeter. I have tried to generate these PWM pulses for the FORWARD command with the Arduino using Bit-banging and when I hook up a speaker to the Arduino, the signals sound identical to TX2 remote signal sounds. With some luck, this generated pulse signal from the Arduino will be understandable by the RX2 in the car and make it controllable. First I will try to connect the TX2 signal output pin 8 directly to the RX2 signal input pin 3. The datasheet of the TX2/RX2 chip has an example testing circuit that does the same, so it should work.
I will keep you guys updated on the progess!