One problem related to wiring has been reported for the 2005 Volvo S40. The most recently reported issues are listed below.
Often, the headlamps bulbs fail, after a few years changing bulbs, I found out that due to contact between detachment of the wiring harness cover is the cause of the failiure bulbs in right and left headlamps. The wiring harness instaled by the manufacturer do not resist voltage and heat.