1971 E-Body Power Window Regulator Question

Greetings: I am replacing the power window motors on my 1971 Challenger CV. In doing so, I've discovered that both rear side windows do not have the coiled helper spring on the regulators. Both front doors have the springs attached to their regulators. Could someone tell me if the two rear quarter windows required this spring. There is a provision on the regulator(s) to attach the springs to. I'm not too certain if the factory left these springs off of the rear windows due to this glass being lighter than the front door glass, or if they are just missing? Any help would be greatly appreciated. Thanks

My 70 has light springs but my friends 71 doesn't...

Thanks for the response to my question....