DS this issue on 2013 WRX has already been discussed recently on a different thread. The fact is hill-assist cannot be disabled on WRX model and is only an STI feature. And hill-assist will only engage if you're in 1st gear on an incline. You can look for proof in the online manual for the 2013 WRX/STI. If you're disabling this feature by other means (cutting a wire or something like that) you'll risk damaging your traction control system.