Unfortunately a second hand unit doesn't work after 1999. They are coded to the car. you need a blank (new) unit, and for it to be coded by a dealer. Many people retire the car at this point, sadly. It can end up costing a few thousand, apparently. 1998 was kind of the last time you owned your own stuff when it comes to Volvo, after that it became locked up in dealer locked down shenanigans, even wacky parts like the heater control.
I would be carefully checking the electrical connection and the relevant supply relay. I had that problem, and it turned out to be a faulty seal on the plug connector for the unit. All good after cleaning and electrical flow was restored ( not noisy).
There are also some kits around, that I have never used, that let you rebuild it. It's not the main board with the coding that fails, as I understand it.
Xmodex do a replacement, maybe still?, but most people have already thrown out your model due to transmission faults that it's not really economical for parts supply companies to make and supply parts for them. You might have trouble finding a replacement part swiftly, other than paying Volvo price, and that will probably be more than the value of the car at this point. It the problem with obsolete modern cars, at a point you can't really fix it anymore. They are no 145….but much safer and more comfortable.
