rear speakers placement?

I noticed on my 1991 Mitsubishi Mighty max that there are 2 vent like covers screwed to both sides of the bottom of the cab directly behind the seats I was wondering if this was designed for rear speaker placement if so what size speakers and if not can I place a set of brand new 6.5" speakers empty vent covered wells. if more discription is needed I could try to get pictures.

Which Ones Are You Talking About?
Top Arrow Or The Bottom Arrow(in Picture)?

I never really looked myself. I just made myself a box with two 12" subs and two 6x9s in it. I do know that the rear speaker are 6 1/2" speakers.

the bottom arrow is exactly what I was talking about but, I never noticed what the top arrow is pointing at being there what is the top arrow pointing at exactly? but yes I was talking about the bottom arrow. which do I use?

Arrow on the bottom looks like some sort of vent. Top looks like a speaker.

I Have Look Down There Before and i think i remember seeing speaker wires but its been a long time ago.
You Would Use the same size as your dash speakers.

all I have are some brand new 6.5" speakers that were given to me as a grad gift are you sure thats not the size for the rear speakers because I have heard from several sources that 6.5" is the size needed for rear speakers in a 91' mighty max. I'm gonna cross my fingers and see if I can't get the 6.5" to fit. I would really hate to have to buy another new set of 4" speakers for in the back as well. When I opened up the vents I don't recall seeing any speaker wires it just seemed like a tiny open compartment not sure though. What is the top arrow in your picture pointing at anyway? I don't recall ever seeing whatever that is in my truck could I place the speakers there?

The Top Arrow Is Pointing at where your 6.4 speakers will go (in Some Trucks)
The Bottom is where your 4" would go!
I'll try to take some Better pictures for you Today!

awesome I am very grateful for all the help. I certainly hope my truck has the speakers wells for the 6.5". if not I guess I'm stuck having to buy another pair of crappy 4" speakers I won't even be able to look until most likely friday when I get the truck back from the shop. good thing I have some 6x9's in a box, I would just really love to have the rear speakers added as well. keep me posted thank you.

Here is a picture of my truck with the 6.5 in the sides and you can see the cover at the bottom.