91 rear speaker problems

I am replacing the rear speakers and found a circuit board back there. What does it do and why isn't there sound coming from the new speakers. Is there a secret set of speakers in the top rear corners of the truck, they look like speaker vents.

Don’t get rid of those speaker boxes! They are some of the most efficient stock speakers!
The box is a tuned bass reflex enclosure. They will provide quite a bit of low end bass output.
The circuit board is a “powered" amp, designed for just that one speaker, all the wires connected to it are: Power & ground, amp enables, speaker wires positive and negative polarity. Replace with a high end speaker of the same travel and you will be better off than any other speaker you can buy.
I just asked at a speaker reconing shop about these yesterday and he said he would not be able to match the quality of the speaker in them.

The grills up in the plastic covering are for some stock tweeters there are even wires in the harness fro them.
That whole piece just “snaps” down.

Thanks 4wheel. That's some good info, I still don't get sound out of my new speakers when I hook them up to the existing speaker wire. Should I bypass the circuit board or is there another problem.I bought an mp3 player and a pair of 4 inch for the front which work fine,but when i fade it to the rear, all i hear is some very quiet static from the new 6x9s.

Where did you connect the 6X9 into the harness? or inside the speaker box?

I connected the 6x9s to the old connectors inside the speakerbox which are connected to the circuit board.

Check to see if you have 12v on the wire that is pink-lightblue stripe. It is the "enable" there is also a 12v and ground wire besides the two speaker wires.