I doubt it's the light switch. When you have a lamp failure system like on these cars, the light switch just signals a control module what to turn on, and the control module monitors the individual bulbs to see if there is a failure or not. So if you turn on the parking lights for example, the control module turns on just the parking lights. it sounds possibly like it's a bad lamp control module, but I'd get some testing done before ordering anything that expensive. The module is in the rear part of the fuse box, you need to take some screws out to get to it (on the way rear part of the fusebox, behind all the fuses).