The Carnetix unit is most likely overheating since it shuts the fan off in standby mode. Once the fan shuts off the Carnetix unit will quickly hit the shut off temperature which I think is 60 degrees Celsius. It's not hard to hit in a vehicle during the summer.
When I was fiddling around with sleep mode, a attached an extra fan to the Carnetix unit and wired it to the battery. It helped quite a bit with the overheating.
Thats gotta be it...is there a way to wire the carnetix's fan to stay on...or better yet maybe like you I should just run a 12V fan with a hidden switch for when I know its gonna overheat.
Or enable safe sleep in case the Carnetix overheats and shuts off.
As far as I know, there's no way to keep the carnetix fan running when the unit is in standby.
Its already in safe sleep where it will resume where it was prior to losing power.
but getting a fan to run off the 12V battery to a hidden switch that I control seems the only solution.
Not sure if you've ruled out using the MacCuff or not, but I've located the Carnetix P1900 and Mac Mini under the passenger seat. First picture below shows an upside down view while I was mounting the MacCuff. In the second picture you can see how it will hang once I remounted the seat. Because of the tight fit in my Pro5 it's difficult to take a picture under the seat.
This option is great for reducing the overheating issue because both devices have plenty of space under the seat, and as you know heat rises in your car so the coolest part of the car will be out of the sun and as low as you can get. This area will also cool off the quickest (with the exception of any areas where your vents are directly pointing).
I use a cinnamon mount in the center console with the mini facing up in the vertical position.