I've done some searching, and ran across two companies who make adapter kits that do exactly what I'm looking for. However, they're quite expensive (~$350, plus the camera), which is more than I'm willing to pay for this. Other than that, this is strictly in the idea phase. Not sure I understand what you mean by "what I already know" - I was just wondering if something like this is possible, and have since found out that it is, but that it's pretty pricey. Is there a less expensive way to tap into the car's screen and have it display what the camera sees?

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2003 X5 4.4i
1992 Porsche 968
2007 328i
2014 Subaru Forester XT