2005 Volvo S60 Truck Electrical lock inoperable

Dont know if this is your problem, but it is a known problem with the remotes...I will email you the full tech bulliten:
KEY INTEGRATED REMOTE CONTROL, INTERMITTENT FUNCTION TECHNICAL SERVICE BULLETIN Reference Number(s): 36-49, Date of Issue:  May 16, 2005 VOLVO: MY 05 - S60, S80, V70, XC70 and XC90 CHASSIS: S60 468280 - 485113; S80 407494 - 415649; V70 500355 - 522330; XC70 193327 - 204897; XC90 190567 - 218294 REFERENCE: VADIS/VIDA repair instruction GROUP: 36 NO.: 49 Related Ref Number(s): 36-49 ARTICLE BEGINNING DESCRIPTION Customers may report an intermittent function of the Key Integrated Remote Control (KIR). This can be caused by a slightly misplaced battery spring clip connection. Potential affected vehicles; Cars with reported complaints of intermittent KIR functions within the above chassis limit • Cars undergoing Customer Preparation Service (CPS) within the above chassis limits • These vehicles shall have both KIRs checked and adjusted. This TNN describes how the KIRs can be checked and corrected.

Look at the harness that is routed on the left side of the trunk. You could have a wire that is going open circuited. Jiggle the harness around while you try to lock and unlock the trunk with the remote. If you do find a problem you can try and open up the wire loom and look for the bad wire or you can remove the harness and replace the trunk harness but that could be expensive.