I think panel voltage is the critical factor when considering MPPT. I was offered a 110W at 18V or a 150W at 28V. I fitted the latter and, looking at the figures on my phone, I note that the current flow into the leisure batteries is often 40 - 50% higher than the current produced by the panel.
A word of caution - I actually painted the top part of the middle two vents matt black as they produced annoying reflections on the windscreen.
The top image shows the effect I wanted to avoid and the middle one the result.
From experience I would suggest that you need a more substantial switch. I used one of these and it only lasted a couple of weeks! You might consider the CBE range as 1) they might well match your existing switches and, 2) there is a Double Throw version available in the range...
If the sun is shining and the panel is not in shade you should get a voltage of at least 17 - 18v depending on the size of the panel. If no reading at all then this suggests an open circuit (broken lead?)or faulty panel.
As AllanM above pointed out, on the X290 the standard horn - a Fiamm unit, is located under the offside headlamp.
Remove the headlamp (two T30 bolts and shove the unit towards the centre line of the vehicle to release)
Now by sticking your head right into the space where the headlamp...