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I have suggested this mod for a while now by removing the rain sensor thats built in the rearview mirror and try converting it to a dash cam,it probably be a little challenging but the wiring should be there to connect the dash cam and who needs a rain senor anyway,that will be my next project,among others.
Garmin makes a really nice one. The only thing that's not so nice is the price. They have two versions. One with GPS and one without.
You go to ebay and get one. Spend at least fifty bucks to get the real one. There are allot of knock offs.