Hooray! It worked! I went from nothing (or 1 bird or a bad accuracy and then a sudden unlock) to a full lock in about a couple of minutes.
I used a smaller wire and did a smaller welding, so the antenna now touches with the cover (ground)!
I couldn't reach an accuracy better than 10mt, however I was on the balcony, so the results on the street may be somewhat better.
That's the difference with Bluetooth external GPS and internal GPS without this fix:
GPS without HW fix
GPS with this HW fix
Thank you so much, my fellow citizen.
I took a 3 hours long walk this afternoon and here's a graph of the accuracy (I never lost signal holding the phone in my hand, although with BT external GPS I can even put the phone in my pocket).
This wasn't possible without this HW mod.
BT gps VS integrated gps while riding my bike. They're both in a mountain bike bag, so that just a little part (where antenna is) looks directly towards the sky.
I guess you can notice by yourselves when I closed my BT GPS and enabled integrated GPS. I was curious to see if I could use it in case of an emergency (ie. dead BT GPS battery).
Left track: integrated GPS
(as you can see, it's less accurated, somehow it does its job in case of emergencies) /// Right track: BT GPS