Yes I believe they are correct. Unfortunately as is the case with more and more vehicles these days, the manufacturers are combining circuits and controls for different vehicle functions into a single module--in this case the radio. GM has been doing this for the last few years, Infiniti is starting to do it, Mercedes/BMW have these, and I know the last few years some VWs have been like this as well.
The actual vehicle functions that require the factory head unit vary greatly from vehicle to vehicle and also within the different model years. Anywhere from door chimes to factory alarms to oil change lights to dome light controls, etc.
There may be a relocation harness available within a year or so but as the 03 is pretty much a brand new model don't expect such a product anytime soon.
Anyways what the relocation harness does is basically it keeps your stock head unit's data wires hooked up to the car, but disconnects the antenna and speaker wires. And instead those are run to the aftermarket head unit, along with power/ground/turn on connections, while the stock head unit is relocated elsewhere in the vehicle (in the trunk, behind the dash, etc.). The stock head unit does not have to be accessible it just has to be hooked up, and as long as that's done the data functions should be normal.
But like I said you need a special "t" harness to relocate the factory radio and I am unaware of one for a VW as of right now. Try www.BestKits.com, they always have the latest and hardest to find kits.
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Ethan
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"Patience, persistence, and perspiration make an unbeatable combination for success"
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