VSS is vehicle speed sensor, doubt its behind the radio. try behind your instrument panel if you know what wire your looking for, if not get the vehicle wiring diagram so you can locate exactly where it would be and what wire it is.
doing alittle research some people are saying its a brown with white tracer and others are saying grey with white and ive also found green with white tracer
you're gonna want to get the actual wiring diagram for you're year truck b/c they do vary from year to year sometimes. best thing is either go to the dealership and talk to one of the tech's to see if they can get it for you or atleast copy the page from the book for you.