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Yes but where are you looking to purchase? Splicing relays into the existing harness will not fix the problem. If you are going to upgrade the PW harness, replace the entire harness with a relay controlled harness. Also replace the switches while you are at it since they are cheap enough. I would suggest the following:Replace the PW motors, clean and lube the tracks, rollers and pivot points on the regulators. Replace any damaged or worn felt seals and lubricate them with silicon spray. The window should travel up and down with ease prior to installing the lift mechanism. I can post a video of the ones I just redid which operate extremely well.
If you are not upgrading the entire harness, then splicing in the relays into the existing wiring may not lead to any benefit. If you run current directly through the feed to the motor do they operate any better?
You would disconnect the switch and run a fused jumper directly to the dark blue wire and ground to the brown wire and reverse polarity to go the other direction. If there is no improvement then you need to check the mechanicals and age/condition of the motor itself. Replacing the harness is too expensive and time consuming for it to have no effect on your problem.