The aux water pump stopped working on my '98 VR6 after 100k. Tested the connector from the harness to the pump and it had 12V when the ignition was on. No problems there. I then tested the resistance on the motor across the terminals and it was open - a couple of mega ohms.
Took the pump apart to have a look and it turned out to be the brushes. One of them was worn to the point of not touching the commutator. When the brushes were out, I noticed that they were similar size to the ones used in 1/10th scale R/C car motors. Not the enclosed type, but the ones where the brushes can be accessed easily at the back of the motor.
I had an old R/C car motor laying around and took a brush off it and fitted it. Put the pump back together and check the resistance and it now measures 3.2 ohms across the terminals. Plugged the pump back into the harness and its spinning happily again.
Hope this information will be useful for others.