As has been said above, AIUI the units are coded to the car so rather than go down that route, buy the relays and replace them.
I did it myself for my S40 and it's not too difficult, providing you've got the right tools in advance.
But before you start taking everything apart, here's a link to a post I wrote about a quick test for your CEM, might help you to diagnose what's happening.
https://www.volvoforums.org.uk/showthread.php?t=272600
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Now: 04 reg V70 2.4 SE 2.4 auto 177k
Then: Volvo, Saab, BMW, MB, Ford, Vx, Opel, Fiat, Rover, Porsche, Alfa, Austin, Renault, etc
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