Wiper motors usually always get +12V and the switch/intermittent circuit grounds the other (-) side . . .
Perhaps on some makes, but it appears to not be the case with Subaru. Attached is the 2005 wiring diagram for the rear wiper where the negative side of the wiper motor goes directly to ground and the 12 V supply is switched. I also checked a number of Subaru wiring diagrams from 1995 to 2011, and they're all the same ground side configuration.
In the attached diagram, FB-23 and FB-24 (at the top of the diagram) are switched 12 V lines that are powered when the ignition switch is at ACC or ON. When the wiper switch is turned to ON, pin C6 of the BIU is grounded. The BIU then connects 12 V to the motor from connector pin B1 to wiper motor connector pin 2. As the motor begins to turn, the internal switch moves from the Off or rest position (as in the diagram) to the other side, connecting the 12 V at motor connector pin 5 to motor connector pin 4, and from there to the BIU connector pin B8.
The motor internal switch remains "On" whenever the wiper arm is away from the parked position. Consequently, there's 12 V at BIU pin B8 whenever the arm is off park. Now, although I'm not sure how this works internally inside the BIU, as long as the wiper switch itself is at ON, the BIU continues to provide power to the wiper motor. However, when the wiper switch is turned Off, the BIU senses whether or not there's 12 V at BIU pin B8, and as long as there's 12 V there (coming through the motor internal switch), it continues to power the motor. But as soon as the wiper arm reaches the park position, the motor internal switch will toggle over to the Off side, opening the circuit from motor connector pin 1 (12 V) to motor connector pin 4 (to the BIU) and grounding BIU connector pin B8. At this point, the BIU cuts the power to the motor thereby stopping the arm at the park position.
In intermittent operation, BIU connector C18 is grounded by the wiper switch and the BIU powers the wiper motor, but in this case only for one cycle. Again, it uses the motor internal switch to signal when the wiper arm is back at the rest position after the first sweep at which time the wiper motor stops running. The BIU then waits a set time, and starts the single cycle over again.
The stopping point for the wiper arm is determined by the motor internal switch that's mechanically connected to the wiper arm shaft drive mechanism, and it's the same whether the wiper had been running with the wiper switch in the Intermittent or On position. (This makes the OP's description somewhat puzzling to me, as it should be the same. See Rear Wiper Does Not return to Park or Home Position
for a previous case.)
Incidentally, the use of the BIU adds an additional feature -- when the rear wiper is in Intermittent operation, if the transmission is set to Reverse gear, the BIU will run the rear wiper continuously instead of stopping after each sweep.