My car won't go in reverse
 

A while back I bought an xmods G35 because it was on clearance and my radioshack was closing. Well when I got it everything worked great except for reverse. The back wheels just twitch and occasionally go forward when you try to make it go backwards. I was just wondering if anyone knows how to fix this. I have pretty good soldering skills if necessary. I searched around a little bit but I couldn't find anything. If there is another thread that tells how to do this just point me in that direction. Thanks in advance.