Denmark gets 1200-1600 hours, Australia gets double that.
We get roughly double the panel output that Denmark does.
Global Solar Atlas
I saw zero measurable gains from 100W on the petrol Kangoo, just as I saw no gain from 40W on my Mercedes.
This was all with roof mounted panels. Mounted inside your car a panel would be shaded most of the time -it only works for that small amount of time that you can drive with the panel facing the sun. 105Wh assumes the panel is angled towards the sun. On a vehicle the ideal angle is zero degrees. So more like 85Wh/day - assuming you only drive into the sun, and park facing the sun.
That's a tiny amount of energy - if you drove all day that's ~8w that you're getting, roughly the same effect as replacing your tail lights with LED. You haven't specified the size of your battery, but by the sounds of it, it's tiny. A 7.4Ah battery has no problem starting my cars. The point being it's not even a case where you can store a lot of power while parked for use later in the day.
My Trafic uses 3.8/100 at a steady 74km/h (lowest speed in top - A/C on 3-400kg on board). 360W gives me 0.2-0.3/100km. Adjusted for a behind glass 50W panel that would be 0.02l/100km - smaller than you can reasonably measure. Bare in mind that 360W of solar panels will never produce 360W of power when mounted on a car -about half the rated output is what you'll see.
If you look at my side bar, I've got all sorts of cars. The LiFe is in the Proton, a very simple car (MPEFI). The improved charge efficiency saw a solid 10% gain. If I connect a grid charger/ solar panel it just switches off immediately as the alt keeps the battery 98% full. Solar panels are pointless in this situation as there's no where for the current to go while parked, same goes for a really small capacity battery.
On the Trafic, with a 120Ah battery, stop start, all city driving and low RPM hypermiling, I found that the alt really couldn't keep up with demand - meaning stop/start didn't work. So now when I park, the power is stored in a large battery - which means stop start works 90+% of the time. The alternator sits at 11.7 volts with the engine running.