In bad weather, or after standing wet for a while, the roof used to dribble from the back corners of the roof headliner.
I had the aerial off over the summer to vinyl wrap the roof on my sportwagon.
The aerial base looks like it was originally softish plastic/rubber but that had gone rock hard and there was quite a large amount of dirt, grot and slime on the roof under the mounting. The hole in the roof is clearly punched in by machine leaving a slight depression to form a puddle, which then dribbles across the inside of the roof skin, or runs along the headliner by gravity to the first available/lower point.
My fix was to clean the area of the roof carefully, and the aerial, then spray it and the roof area with silicone spray. That should help keep water out. If it wasn't vinyled, I'd have added some silicone sealant to the join too. The bolt that holds the aerial on the roof was very loose when I removed it, so I tweaked it up as tight as I dared. No water dribbles or musty smell yet.
Might be that.