One problem related to wiring has been reported for the 2004 BMW 525. The most recently reported issues are listed below.
Driving down the road and smelled a burning smell. Turned off the seat heater. The seat felt very hot on my lower back. When I got to work noticed that the leather was burned through on the drivers left side. You can see the exposed wiring. Very dangerous. BMW would not replace. In searching on line, I saw that this is a common situation on the seat heater element.