Are they supposed to blow much weaker than the front vents? I just had the AC on with people in the back. I had the AC on pretty high.....I was freezing and they were still hot. I felt the vents and it was blowing cold air, just very weak. The vents were fully open. Is this normal?

 

This has shown as a common problem by many. I am going to solve this in the future when I get the free time by putting a computer fan in the vent duct near the rear vents to pull more air from up front/push more heated/cooled air into the back

 

Another way to address this is:

Carefully aim the center dash ducts so the air passes between the front seat folks and heads right on toward the back seat, giving them a tad more cooled air.

Close the vents on the ends of the dash, hence adding pressure to the air going thru the small duct to the rear seat outlet (this also blocks the cold air from those dash-end ducts bouncing off the front door windows and hitting the front seat folks).

I have had good luck with that method, but if that is still not enough, then:

Close all front dash vents and have all the air go to the back thru the duct to the rear seat outlet. If I did this, I would not have the fan speed too high, as the backpressure from having all the main ducts closed will cause considerable backpressure, increasing work and wear on the fan motor.

I have fought this problem since air conditioning was added to cars around fifty years ago, and the above approaches have been the only things that have somewhat worked for me.