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I've had my car since 2002 and have always carried a spare. It's saved me twice. Do like the others said, get a new one and put it in and use the old one for a spare and replace it as soon as it fails so you've always got a backup. Last time mine failed I was probably over 1,000 kilometres from the nearest dealer. It's cheap insurance and do the other one too, replace all your belts and keep the old ones for spares in the car. Do a search and you will find a recommended tool kit to keep in the car.
I carried a spare DME for over 6 years. Some month ago in the middle of the wilderness with no cell phone reception it failed and I replaced it. Do I carry a spare? Sure thing I do.
The $20-something DME relays are slightly taller than the OEM Porsche parts, and the cover does not close as well due to the extra height. Keep that in mind.
I drove my 993 the first 100K miles without a spare DME, all around some pretty desolate parts of the western states.
Then I realized I had been pushing my luck, so I bought a spare, and a bunch of other assorted stuff for $200 from LA Porsche Dismantlers. Never had occasion to use any of it.