Has anybody tried a 6v to 12v converter? The one shown says it is adjustable and there are other types at the bottom of the page.
I used this one to run my GPS and charge my phone. Cant do both at once but really cheap. You have to supply your own enclosure and wires.
https://www.amazon.com/gp/product/B00C9179WK/ref=pd_sbs_23_1?ie=UTF8&pd_rd_i=B00 C9179WK&pd_rd_r=ZD34XEP8T9MW0H3PT975&pd_rd_w=zsT5G&pd_rd_wg=vj8GN&psc=1&refRID=Z D34XEP8T9MW0H3PT975
You don't need 12 v to charge a phone or a GPS - the standard voltage in their batteries is usually about 5 volts, so most of them will charge just fine if you hide a lighter socket somewhere to plug in a charge cable from the device.
I run my Garmin GPS on 6 volts. It has turned off when the battery was low on idle and started again at speed, but since I increased the charge at the third brush so it didn't run low with the lights on, then it hasn't turned off.