RepairPal Certified shops can diagnose and repair your 1991 BMW 525i. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.
Looks like its located on the firewall at the middle of the engine compartment, Here a online guide to remove and replace http://www.autozone.com/addVehicleId,1601901/initialAction,repairGuide/shopping/repairGuide.htm?pageId=0900c1528004a280