Dave Burton (Volvo GOD) / top Volvo relay vendor in the Internet has this page for your reference,
LINK
HERE
The relay tray is under the empty center console tray. //// mounted side way.
(picture on the bottom)
The reply and relay socket question is label as relay "C"
Check for burn on the base of the relay,burn smell,corrosion on pin(s),and bad socket base.
Give the relay "C" a firm press to re-sit it.
If it does no work then you will need to do some "known good" relay swap to see does the trouble follow.
I recommend you to re-sit and inspect each relay before do the swapping.
Keep a good record on which relay you are swapping around.
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Also try your 2nd / spare key fob remote to see does it unlock the doors.
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Last item.....never assume, Check the battery voltage at the battery posts.
It should be 12.8V +