Question on a car kit I bought
It seems that you'll have to do some extended modifications to the design. In fact, all Arduino Pins (D0 - D13, A0-A5) are connected to components on the board and thus in use. 

Schematics and Code: https://mega.nz/#F!Xbg1FA5L!qGah_HlLFJ_NKxv7ILLY2g folder "5 SCH".



This means that the only way that you'll get new components into this project are:


removing components  and re-programming the firmware to not use them


e.g., the two tracking-sensors left&amp;right -> A2 and A3 are free 
e.g., the speaker pin (D13). Who needs a speaker? 

restructuring components


e.g. for the analog sensors: instead of using the Arduino's analog pins, throw a I2C ADC module (e.g. ADS1115) in the build and change the firmware to get the analog values via that new module; making this project use an I2C bus will free all analog pins and cost 2 pins (SCL + SDA). The I2C bus has the additional benefit of being able to connect more than one device to it -- you can e.g. put a I2C GPIO expander (e.g. MCP23017) on it to gain 16 more digital I/O -- more than the Uno had to begin with.