Rear window defogger on 94 E420

[screedchan]

I am having problems with the rear window defogger on my 94 E420. When I push the switch to turn on the defogger, the indicator lamp lights up, but does not stay on when I let go. It seems to me that it can only be one of two possibilities, either the switch or the relay which controls how long the defogger stays on. My money is on the relay, since the indicator lamp lights up when I push the switch. If it is the relay, can anyone please tell me where it is located? If there is another possibility, I am always grateful for any thoughtful response. Thanks.