[groar]

If their width isn't very important, then you may put them over the dash board under the front windshield. Preferably on the passenger side to avoid reflection in front of the driver.

If you put them against the windshield, in case of emergency stop they shouldn't move, or if they move they shouldn't be as dangerous as they would be under the back window.

Anyway, photovoltaic doesn't generate a lot. Here where I live (New-York's latitude) in summer you can have only 7x peak power, ie 7x5x2=70Wh which is 5.8Ah of a 12V battery, ie a tenth of a standard battery (and I didn't included losses such as voltage mismatch, cable lengths...).

Also be careful that if your battery is charged then your solar panels will push up the battery voltage over the limit (14.6V) and deteriorate it, you will then have to manage the water level of your battery. A small 1A charge controller should protect your battery, noticeably when you don't use your car during several consecutive days.

Have fun,

Denis.