Re: Silly Question!!!!!
The aerial in the 156 is integrated into the rear window demister element, then goes to an amplifier behind one of the c-pillar trims (drivers side in my old 156 saloon). It works well enough with strong signals but isn't as good as a roof aerial on weaker stations.
Don't know about the tax disc holder though, not seen that before. Does it look like a coaxial RF type cable, I'm just thinking mobile phone or GPS antenna? Hopefully someone else will be along soon.
And don't worry about asking silly questions, you should always get a sensible response here, at least on Sunday mornings, Saturday evenings maybe not so much :-)