For some reason, even though I place the vehicle turret into my car's inventory, with plenty of ammo which is NOT loaded into the car's machine gun, the turret refuses to spawn. Nothing happens when I get in, out, or in combat. Is there a key I'm supposed to press or what?

The turret should spawn almost immediately after you put it into the inventory. It's not a bug I've ever encountered myself, so I need more information to help you.
What other mods are you using?
Is the vehicle you tried to use from the base game? And if not, did you place the vehicle down, or just load into the game with the vehicle already existing?
Does your game have custom forces?

Well, I'm using several mods. perhaps it is an incomatability with AAI Programmable Vehciles. also, I have no Idea what qualifies as a custom force.

Full list of (active) mods:
AAI Programmable Structures
AAI Programmable Vehicles
AAI Signals
AAI Vehicles: Flame Tumbler
AAI Vehicles: Hauler
AAI Vehicles: Chain Gunner
AAI Vehicles: Laser Tank
AAI Vehicles: Miner
AAI Zones
Aircraft
Additional Turrets
Arborium
Bergius Process
Bullet Trails
Data Raw Prototypes
Detatched Gun Sounds
Electric Vehicles
Electric Vehicles Lib
Fast Long Inserters
Flow Control
Free Real Estate
Honk
KS Power
Mudraker's Terraforming
Nuclear - Nuclear Power
Off Grid Effects
RailPowerSystem
Rail Tanker
Renamer
SP Respawn Mod
Smarter Trains
Space Extension Mod
Subterrain
Surfaces_Remake
Trash Landfill
Vehicle Wagon
Vehicle Turrets (That's you)
Warehousing Mod
Wireless Charging
Wireless Charging Lib

Yes, I am a complete mod junky. With so many mods, I doubt there's much you could do to help, but anything is appreciated. I have absolutely no idea how to use AAI Programmable vehicles, the only reason I keep it around is because it makes your vehicles act like sentries when you're not in them.

Wow, that is a lot of mods :P I'll at least try the vehicle related ones, but it's gonna take some time. You never answered if you used a vehicle from the base game or not (Car and tank). The turret will not spawn if it's added to a vehicle from a mod, if that vehicle was already placed when my mod got installed. It should work for any vehicles placed after the mod has been installed though, but could be some mods implement vehicles in a way I didn't expect.
I'll see if I can find something.

It is indeed AAI Programmable Vehicles that is the problem. It's a big mod, so I haven't found out exactly what it does and why, but it seems like it replaces any vehicle you place with a different vehicle, removing the reference my mod needs to have to the vehicle.
I should be able to fix it so that it gives a new reference when you enter the vehicle, but I want to see if I can find exactly what happens, and see if I can make a more fitting fix.
Without that one mod, everything works though. Turrets can be added to all the new vehicles like the aai laser tank, and even the aircrafts, which I found hilarious :p

That turned out to be a lot harder to fix than anticipated, as that mod really likes killing vehicles and spawning new ones, but it works now =)

There is one small issue though, if you mine a vehicle that's not from the base game, then you will not automatically mine the active turrets on the vehicle. It works if you mine the turrets first though, and worst case scenario you lose 4 turrets and 40 ammo :p
I'll fix it tomorrow at some point, it has gotten too late now.

I hope you'll enjoy my mod =)

Dude, you are the best. Aslo, I realise I forgot to specify, and I was indeed using the vanilla car. On an unrelated note, was the tank debuffed? I do NOT remember it using fuel so quickly, and it used to have a machnie gun slot

That is not my mods doing :b While checking up on what aai programmable vehicles did, I did notice that when it removes the placed tank, it replaces it with a tank with only a tank cannon. It also changes a lot of variables related to fuel consumption. My mod is innocent there :D

Ah so programmable vehicles is to blame! Still though, I can command a fleet of tanks into battle and just watch from the sidelines, so I may keep it, even if I only know how to use a fraction of it's full power