I will hazard a guess and say your unit does not have one.
This is not a bad thing as it gives you the options of what you want to use.
My only suggestion at this point is, do NOT install one inside.
2016 F350 6.7L LB CC Reese 28K 2014 Chaparral Lite 266sab
"Those who would give up essential Liberty, to purchase a little temporary Safety, deserve neither Liberty nor Safety." 2014 19 days camping 2015 17 days camping201620 days camping