The factory ones last 20 years before breaking, I have done this on my old 240 wagon and recall the wiring hardnesses were fairly cheap, maybe $80 each genuine delivered. Installed and good for another 20 years and looks right - mainly because it is right.
You may have luck with Carnut, he may have some second hand ones in the 42 or so 240 wagons he has in his many sheds...
If you are dead keen on making it yourself, replicate the original, Volvo engineers were pretty handy with desiging long lasting cars back then so probably did it that way for a reason.
btw, allow for 4 wires a side from memory. elec motor for wiper, tail lights (number pate), brake light, central locking, probably an earth, rear demister on the glass and probably a big earth for that one, guessing 15 amps on that alone. That is 5 little ones and two big ones.
Hope this helps