1992 loc install

hello, i know there are million threads but i need to ask one question
im about to buy a high low adapter with harness and i have a external amp
do i plug the harness behind the stereo or at the amp

both places have the same 2 plugs, but the stereo itself only has 1 plug, they dont say on witch location you plug the harness in

here is a pic of the adaptor


thanks from Iceland

Hafsteinn