I have just traced through the antenna wiring in my car (2007 Golf GTI).
The white connector goes to the rooftop antenna, the beige connector goes to the rear window antenna. Neither have amplifiers or filters in line. Both will do AM & FM, all though the window one is better for AM and the rooftop better for FM (in my case - it probably depends on your location). There are no modules or smarts in line, they are just direct connections from the antennas to the head unit.
The unit itself (RCD500) had the antenna amplifiers & diversity switching circuitry built into it, which is why people have so many issues when they change to an aftermarket head unit.
Note: My actual findings don't match up with Elsawin. It talks about a module in the C-pillar for the rear window antenna, which simply isn't there. It also mentions a diversity switching module under the right hand front seat, which also isn't there.