Looking at the 940 wiring diagrams for you, at the tailgate:
o Black is to chassis ground behind the left taillight (uses the same ground screw as the taillight assembly).
o Yellow-white is +12V for the wiper motor from the wiper relay (relay socket pin 6, relay pin "S"), controlled by the wiper switch.
o Orange is +12V for the wiper motor park function. It's the same power the relay and wiper switches get from fuse 14 when the ignition is on (socket pin 4, relay pin "15").

Verify that the black wire provides ground (screws on the aluminum tailgate opening threshold are a convenient test point). If you don't see power back at the orange wire in the tailgate and everything else on fuse 14 is working (like the radio) then it's a wiring issue. Unlike the 140/240 wagons, the tailgate wiring at the hinge area in 700/900 wagons is generally not a trouble spot. Double check that the wiper wiring inside the tailgate hasn't somehow been chafed by the wiper linkage.

My main initial suspect would be the rear wiper relay (back row, rightmost in the tray), although I don't recall that I've had one go yet. I'd be able to swap in the same relay from my other wagon, but you likely don't have that luxury. For you, diagnosis will be a bit trickier. Next suspect after that would be the wiper switch. My third would be the motor. That's the order I would normallly want to do for diagnosis, but ease of access to connectors to isolate the fault is a consideration in where to start.

As you've got the tailgate open, you could try jumping 12 volts from the orange wire to make sure the motor runs. Any chance you can get a U-shaped paper clip into the back of either connector half to jump power from the orange wire pin to the yellow-white wire pin? If not then you could open the connector and use test leads to provide ground to the black pin and power to the yellow-white pin to run the motor. Once the wiper moves a bit you can reconnect it and the wiper should then finish its cycle and park. Easier though, once you've got the relay out, would be to jumper pin 4 to pin 6 in the relay socket to accomplish the same thing and force the wiper to run.

For the sake of simplicity and laziness in your case, I'd probably next think about opening up the wiper switch console and verifying the switch, unless the connector there is too buried for easy access (I forget at the moment). You want to see continuity between switch connector pin 1 (orange) and pin 4 (yellow) when the wiper switch is on (to the right). With the switch connector open, a jumper between orange and yellow there should now run the wiper if the switch is bad. Alternatively you could check the switch at the relay tray socket (continuity between socket pin 4 and socket pin 5 when the wiper switch is on, corresponding to relay pins "15" and "31b"). If it's the switch, try a spray contact cleaner.

After that it's time to diagnose at the relay (back row, righmost). For access you'll want the storage shelf out and the tray unclipped and slid forward. Verify relay socket pin 1 (corresponding to "31" on the relay) is chassis ground (black wire on the cigarette lighter is a convenient ground test point). Verify near zero Ohms between relay pins "31b" and "J", otherwise bad internal contacts. If there's an open circuit between relay pins "31" and "86" or between pins "15" and "J" then it could be a fried relay (check +/- both ways in case a diode is in the way).

Although these relays are not known as repairable, next thing I'd do would be to open it up and try re-flowing the main solder joints, being all those solder joints connected to pins "15", "31b" and "S". Nothing to lose by trying. Please post back if that fixes it.

After that it would be time to get a bit more creative in testing, such as carefully wire wrapping test leads to relay pins and re-inserting the relay so you could use a meter to see what's going on and simulate the switch in various positions. Or else go seek another relay (new or good used).

For relays, the front wiper relay lives next to it. It's a similar, but not an identical relay and setup. The diagrams show the relay basic pin functions and socket pin numbering are similar. I've not done this, but there's a chance if you can put the front relay in the rear relay socket that the main wiper on/off function might work, which would prove it's the rear wiper relay. A bit risky perhaps so I'm not advising you try, but knowing me I'd probably try it, maybe after doing a bit more verification in the wiring diagrams and looking at the relays themselves.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now