How did you wash it? Was it at the car wash with the high-pressure wand? If so, there's no telling what you trashed. Modern cars cannot be washed in this manner.
You need to examine all the ground points and any connectors under the hood for tighteness and water intrusion.
Ed in San Jose '97 540i 6 speed aspensilber over aubergine leather. Build date 3/97. Golden Gate Chapter BMW CCA Nr 62319.