hey guys and girls. i don't know much about car audio, however i investigated the lack of sound coming from the right rear 6.75. turns out the speaker is not even there!

my dad has a grand prix with a pioneer HU and polk db series speakers all around. he's giving me his extra 4 inch db's for free for the rear, but what can i do about this missing speaker? we have extra stock 5 1/4s from the doors in the grand prix. could i temporarily rig them up in place of the 6.75s? my car has a kenwood HU btw.. looks like a 4 year old did the wiring

so many little surprises with this car.. thank you in advance.