I recently replaced the central control locking unit of my 2005 C70 with a unit from a breaker that came complete with 2 key fobs - try "Minty" on this forum
{His contact details can be found here
https://www.volvoforums.org.uk/showthread.php?t=2939820}
The C70 (850 based) uses the same unit 1998 to 2005. If you buy a second hand unit, that unit should come with the fob(s) which will be coded to it. The unit is a straight swap over and takes only a few minutes to do depending upon how easy you find it to get in under the dash. The unit you are looking for is shown in the picture below.
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To check which unit you need use a mirror and torch to establish which unit you have - if you slide off the unit from its bracket without removing the multi-plugs it has just enough slack on the cables to be able to read the part number which makes sure you get the right replacement - you don't even need to slide it back on the bracket - it will just hang there on the cables until you can get a replacement. It is easier to plug connectors in to the replaceemnt before you slide it back on the bracket.
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V70 SE 2004 2.3L T5 Geartronic, C70 2005 2.0T Automatic, Sold 2.4l 20 Valve Automatic Torslander Sold 854 GLT 20v Auto