If you wire through the backup light it should come on only when you are in reverse. The other option is wiring it to a switch so you can turn it on any time; driving, in park, not only when in reverse. It just depends how you want to use it and if you want to wire a switch or not.
Taking it for a test drive will do a lot for your peace of mind. Run the vin on toyota.com, get a carfax. Not all used 4Runners are equal. The dealer isn't as important as being comfortable with the purchase. Buy the warranty if you are that worried.Good luck.
