Did you get the same voltage on both of the wires? One should be purple/white and the other black/white. The power window motor circuit is controlled by the Body Electronic Control Module, and will shut off after .3 seconds of inactivity. This is why you're seeing the jump in voltage and then nothing. Also, you're not going to get an entirely accurate reading because of this. 5-7 volts sounds like a good reading, assuming it's the same on both the purple and black wires. The reason that i say it sounds good is that 5-7 is about halfway between B+ voltage and 0. The circuit is closing and opening too quickly for the voltage meter to get a good reading, so it shows an average. Some voltmeters have the ability to capture the highest voltage over a period of time and display that instead of an average. If yours has this feature I would try reading it that way and see what you get.
When I say that you should have the same voltage on both wires I mean that one should have B+ voltage when pressing the switch up and the other B+ voltage when pressing the switch down.