So we have had our 65 Chevy Chevelle wagon since 1991. It has been sitting in the back yard for the last 17 years while I worked and played with other things. The power rear window has also been a sore shop in it's membership of the the family. I believe that most of the issues is with old wiring, switches and the grounding of the motor. Motor originally grounds to tailgate - witch grounds to the body through the then and now rusty winches and then to the battery through decades old rust body grounds. Second thought is power going through the switches then to the motor - amp draw amp draw and more amp draw. So I have gone Euro on the rear power window wiring: One power 10ga fused wire from battery to motor One 10ga wire from motor to cleaned drilled frame connection One 5 pole relay wired in tailgate for up One 5 pole relay wired in tailgate for down So when I use the dash switch to operate the rear power window, the switch is not powering the motor just activating the correct relay and power is there and strong. So now all I may need is a new or rebuilt motor. As after a few years, 54 to be exact, it may need brushes and the comb turned.
That is an interesting way to make the original wiring last a lot longer. It would be interesting to see, however, if there's a modern motor that could be swapped in, perhaps with minor mods or not.
Andrew the biggest issue I see is the poor quality of replacement parts flooding the market today. Especially when it comes to electrical switches and such. Most of the so called OE replacement parts companies sell the same "made in China" stuff that only looks good but function is a totally different subject. So having the amps needed, making it through the switch and back to the motor is problematic to start with new.