I've read multiple articles about diagnosing the cause of faulty wipers but I can't find any that seem to describe the symptoms I'm seeing. Here's what I am experiencing:
At high speed: the wipers start easily and look to be running at the expected speed. When I stop them they struggle to return to the park position but just manage it.
At low speeds and intermittent: the wipers struggle to start moving if it all. A spray of screen wash will typically reduce the friction enough to get them started but it's not consistent and they are jittery. When stopping the wipers they won't return to the park position, they just get stuck at the point I switched them off.
I can't tell if this is a motor issue or switch/relay issue. A lot of articles just talk about the wipers only running at high speed, however here I am experiencing not enough torque in the motor at the lower wiper speeds.
This is exactly how a motor behaves as it is failing. The fact that the behavior is different depending on the motor speed tells that, the motor is not able to supply enough torque with the current that it is getting.
Though it is surprising having a motor fail on windshield wipers. Maybe you could post a picture of the old motor once you replace it -- so we can analyse what might have caused it to fail.