Yeah, you sure do need your detector!! I'd start by looking for fuses...go to the fusebox under the hood and see if maybe they grabbed power for the detector there...if so, follow the line until you find its fuse.

If this fails, I'd start by pulling the trim panel under the dash, above the driver's knees...it's likely stuffed up in there.
I really don't know if the K40 even has a power switch...it may not. Boomer McLoud is a retailer here in New England; they sell K40s so I'm sure they can offer advice too. Good luck, and enjoy your Ride!!

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Best regards, Michael
'92 500E
'88 300TE