I didn't think lead free solder was used much at all until 2006, but I was mainly working with consumer electronics around then where anything with a CPU or GPU connected via BGA started failing really quickly. Maybe it's possible Volvo was trying to be environmentally conscious before there was good knowledge on how much more prone to cracking lead free solder was? I found a thread started by the creator of the Knocksense talking about a 850 fuel relay he repaired.
http://www.edaboard.co.uk/is-this-due-to-rohs-solder-t519002.html
Either way, repairing it lead or silver based solder as Philia Bear said should make a good repair, but you will have to wait 10-23 years to see if your soldering is any better ;)