Thanks for all of the replied guys.
I know the solution is a little overkill. an Arduino is used in so many projects where a "simpler" often single chip solution would suffice. Its just easier for me to understand this way due to the availability of online resources.
I know for example that even an 18650 is too much, i need the circuit to last for literally 15 seconds at most after power off of the vehicle, but again its just familiarity and availability of the components
@Paul Price - Hmm OK, i was trying to be careful not to overcharge (or do anything else weird with) the 18650, hence just going for a standard LiPo charge PCB. But if that would work it would save on components, the Adafruit module is reasonably pricey for what it is. What would a diagram of that look like?
@sokoloff - The car is a CAN based vehicle yes, however the year of my vehicle doesnt have the "total closure" stuff implimented, neither did the later cars to be honest. The later vehicles can roll up and down the windows by holding the lock/unlock button on the fob, but again, mine doesnt have this. Its not something that can just be switched on or off in my model year, its a physical difference in the body control module (much more space available in the later years flash memory so more features were added).
@bob225 - Its a 2004 Nissan 350z
@BillyD - I too have thought about that. The motors themselves dont care if you hold the window switch Up when the windows are already raised. The particular car has frameless windows and as such the end of travel is defined by a limit switch, so there will be no ill effects there.
I do need to investigate what happens with regards to if something is trapped in the window as the OEM system reverses the glass if current draw is too high (assume something stuck in the window). This MAY still be functional as you can power the windows still for 20-30 seconds after switching the engine off, so id assume that from factory that the safety feature has to be available at all times and as such should still work as my circuit will essentially be replacing the in-cabin window switches
So in conclusion, although what i have proposed is way overkill, there are no fundamental issues with is. Its not required to have any capacitors to smooth outputs anywhere etc? and should work if i connect everything up right? I could breadboard this one up, and it it works then try to pair components down to simplify the circuit.
I know its all kinda silly, but i just like doing these things rather then just resorting to eBay if possible. I find it fun and i think this is simple enough to be within my realm of possibility!