I am trying to connect my mini-z car to an arduino mega board so I can read and change several parameters. (a mini-z is a small RC car running on 4 AAA batteries)
I've had a look around and I think (only a guess) that its a one-wire serial link - ie RX and TX share the same signal wire. Try connecting a 1k resistor from TX to the signal wire, the signal wire to RX.
Then you ought to be able to send serial data via the resistor while the other end is in input mode, but not damage anything if it sends at the same time. Stuff you send will be echoed back of course.
You can start experimenting with the chip taken out of the Arduino - this time connect the RX pin via a 1k resistor to the signal line and the TX pin directly to the signal line (so the USB-serial chip can talk direct). In fact having a 1k resistor on both RX and TX pins allows both the Arduino and the USB-serial chip to talk down the line...
Quite what to send (and what baud rate)? - try reverse-engineering the Linux version of the ICS software? If there's no documentation that is.