Normally the relay you're looking for is located in the fuse box under the hood of the car. That would be the battery junction box, in the car under the dash is the central junction box, and I'm not sure how yours is equipped but there is sometimes a body control fuse box under the back seat (although I think that's more of a 5 series thing). You may want to try cleaning the cables and connections to the starter (replacing the relay is cheap and easy so why not?) and get your battery/battery cables checked, make sure they're clean and tight. If all that fails it may be time for a starter or ignition switch. How heavy is your key chain?