I don't know about Subies, but...

For my Honda, they actually make a plastic piece to cover the stereo hole. It's intended to snap into the dashboard on cars that are sold without a stereo.

I ordered one of these pieces from a Honda dealership online, and it snaps perfectly into place over the empty empeg sled. I had to modify the plastic connection points just slightly to make it fit the sled, but after that, it was perfect.

When the empeg is removed and this cover is snapped in place, it looks like there's not even a stereo in the car at all. My speakers are all under the factory grilles, so you can't even tell I have aftermarket speakers. It's a totally stealth installation. Nothing visible to steal==no break-ins.

Investigate this with your Subie dealer.

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Tony Fabris