I want an aftermarket GPS unit that I can mount on my dash that will also directly interface as a radar detector as well as my boost gauge. I know this can be done but I don’t know how or where to start. Anyone software, programs, etc that you guys know about please share.

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2004 Audi A4 1.8t Quattro Ultra Sport. Goodyear Eagle F1 All Season, Magnaflow Exhaust System

There is no dash mountable GPS unit on the market that will do what you want. Now if you want to replace your factory radio with a monitor and build a car PC I could guide you to a few websites that will show you how to make data interfaces, design software, that will allow you to do what you want. Be prepared to spend thousands.