Hi and welcome
A good investment would probably be a VIDA setup, its the proper volvo diagnostics and has guided steps to fix/diagnose pretty much anything. It would be able to identify if the module is present, connected and whether the seat element itself is intact.
Do the driver and passenger seats match well? The R-Design seats can age badly and its possible someone swapped a worn drivers seat for a replacement without heating fitted (I believe it was on optional extra on R-Designs).