How do you know if you need a relay to bypass the parking brake when installing a double din dvd player?

I've been reading alot of threads saying how to bypass the parking brake in order to have your dvd player play while driving (keeps passengers entertained). When do you need a relay? Or is it just assumed with the newer models? And has anyone heard of Sound Storm Laboratories DD405 or Planet Audio stereos? Both are double din dvd players and they look exactly the same. Scams? or junky generic sellers?