first of all open the remote of the Rc car. You will analyze that there are four switches. when we press the button each switch is closed.. Before pressing the button turn on the remote and check the voltage across the terminals of the switch it should give a value.. after pressing the switch hold the button and check the voltage it should show zero..
this concludes that our remote is working on short circuit technique. that is if there is a voltage across the switch and when its pressed it is short circuited.
Observe that one wire is common in all the switches its the ground of the circuit. which concludes that we need to solder five wires in the remote. one for each switch and one for ground.
it can be checked that when u short the ground wire with any of the switch wire the car performs the action of the switch who's wire is shorted if all the four functions are performed then it means u have picked the right ground. and the right switch wires.
here are the pcb design and the simulation of the circuit for interfacing (download parallel port circutary)