At this stage of your diagnosis the problem could be either a dirty/worn RF switch at the driver door pod or a broken Red or Blue wire, most likely at the driver door hinge.

There's one more test you can do. Lower the RF window part way. From the driver door switch, if the window can still go up then you can be sure it's the switch, otherwise you need to do further investigation, as follows.

With the driver door now opened up, locate the Red and Blue wires for the RF switch. Follow those wires down to closer to the hinge area and give them a good pull to see if one is broken. If a wire is broken it may still be held together by the insulation, but a good solid tug usually finishes the job.

If that doesn't identify the problem then you need to open up the driver door and break out a multimeter to do electrical testing.

First off, know that the window switches act as a flip-flop. In the neutral position, neither the Red or Blue wire will have power or be grounded. In one switch position, Red will have +12V and Blue will be to ground. In the other position, Red will be grounded and Blue will have +12V (to operate the motor in the other direction). This is true whether you use the driver door switch or the passenger door switch (they operate in parallel).

Remove the connector for the driver RF switch. Probing into the connector, with your black test lead on the black wire (chassis ground, which we know is good otherwise no switches would work), put the red test lead to the Red wire. With the ignition on, operate the switch at the RF (passenger) door in both directions (the window will of course go up or down). If you can see +12V at the driver door Red wire in one of those positions then you know the Red wire isn't broken, otherwise it's broken. Now repeat that test for the Blue wire to see if it's broken.

If you could see +12V at both the Red and Blue wires then it's almost certainly the switch. You can do further testing on the switch terminals if you want, but there's not much point of you did your testing correctly. To test the switch itself, you would be operating the switch in both directions looking for any continuity between switch pin 5 (for the black wire) and switch pin 1 (Blue) and then pin 5 to pin 2 (Red).

If you identify the problem as a broken wire, it will almost always be at the driver hinge area unless you've been disturbing door wiring elsewhere. You will need to pull open the rubber boot for access and inspection. For the repair, use a sleeve crimp connector rather than solder. Push the sleeve into the pillar area when done so as to move fresh wire into the flex area. You can splice in extra wire back further in the harness if you need more length.

If you identify the problem as the switch then it's often worth trying to give them more life before replacing them. It's usually the driver window switch that wears out first as it's the most used, but the RF switch is sitting out more exposed to wet and grunge, although that's not as common as with the console heated seat switches. Remove the switch from the pod and take it to the bench for disassembly. Carefully open the switch by spreading the sides a bit so you can pry up the rocker cap. Do it with the switch upright so as not to loose the tiny parts and open it slowly to see how the bits fit together, which includes the spring loaded ball point plungers and the contact bars at the bottom. Sometimes it's just dirt in the bottom causing the problem, which can be carefully cleaned, preferably followed by spray contact cleaner. Sometimes the contacts are pitted or worn, either the contact rocker or the contacts at the bottom of the switch, which you can attempt to clean up with something like an ignition file or bit of emery paper. When re-assembling the switch, add a drop of dielecteric silcone grease (ignition grease) to the top of the rocker contact to act as a lubricant for the spring loaded plunger. Re-assembly is a bit tricky and dielectric grease can also help hold bits in place.

If you need to replace the RF switch, note that in 900s the front window switches are illuminated (in 700s they're not illuminated). It's strictly an internal connection, there are no additional wires used for illumination, you can use a 700 switch in a 900 and vice versa. You can replace a driver door pod switch with with any other driver door pod switch (700 or 900). You can also replace it with any switch from another door (such as a rear door you don't care about), but you will have to make a modification at the switch to bridge between pin 5 (black ground) and pin 3 (normally unconnected at the driver door switches because they are internally bridged, you can tack solder a wire to the #3 pin stub). You cannot use a driver door pod switch at any other door (because they are internally bridged). I've never understood why Volvo decided to use three slightly different switches when only two or even one (illuminated, unbridged) would do. That's useful to know when going for a used switch from a yard. If you want a driver door switch they will usually only sell the entire pod and it won't be cheap. If you ask for a switch from a rear door (which will be lightly used) it will be cheaper and easier to come by, you just have to remember to bridge pin 3 to ground and of course it won't be illuminated. If you borrow a rear door switch to use at the front, you will want to put the faulty switch back in that location for looks, just don't connect it (maybe tape a note inside as to the reason).
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now