The wiper switch is rarely the problem.

A problem with intermittent wiper and constant operation does suggest it's the relay. The relay is as you note, hanging down behind a black metal panel located behind the bump of carpet on the very left of the driver footwell where you rest your foot. Rip up the black plastic threshold trim and remove the plastic side panel for proper access. The rubber vent knob pulls/twists off the lever and I vaguely remember early ones had a retaining pin. In your case with a wagon, both the front and rear wiper relays are there. I forget which is which, but a wild guess is the upper one is the tailgate wiper as I vaguely recall it wasn't intuitive -having said that I'm now likely wrong. You will either need to check wire colors or go by trial and error removing one. Volvo p/n is 6849780 for all 240/260/700/900 years, superceding p/n 3523610. I thought the rear intermittent relay may have been a different p/n, but a quick search suggests it's the same, which makes sense and it was likely the superceded factory p/n number I saw.

Simplest for you is switching the front and rear wiper relays to see if that changes things. If the rears now work properly and the fronts don't then obviously it's the relay

When you remove the rear wiper relay and in case you see a different part number from the above then I'll mention that the later 245 DL wagons did not have intermittent rear wipers, yet did have an intermittent position on the stalk switch. The difference is simply the relay, all the wiring is in place. The earlier GL wagons did have intermittent rear wipers and I chose to swap in a (used) GL relay when I was replacing the front relay. The above relay will now give that functionality in case you never had it before and just assumed it was a faulty relay.

Also thinking of common 240 wiper issues, I'll mention if when you turn the front wiper switch off and the wipers don't continue to the park position then that's normally an electrical problem at the wiper motor in the gearcase housing. There will be a similar issue at the tailgate wiper motor that would prevent wiper parking. Note that the wiper motor housing is not grounded, but rather mounted in insulating rubber grommets. There's a copper contact strap under the cover that eventually may not be making good contact. Remove the cover, shine up under the cover and the screw areas, maybe tweak the copper bar up a bit and close it back up making sure the window in the waterproofing plastic gasket allows the copper strap to contact the cover.

It's possible, although not common, that constantly running wipers is a problem in the motor with the contacts that ride on the drive cam. It would pretty much have to be one contact always touching another. As needed, these contacts can be cleaned up and tweaked, just keep everything in original alignment on the cam (maybe make a couple of felt pen marks and take a closeup picture).
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now