I carry two, one is just a large Ziploc freezer bag with really basic stuff tucked into one of the rear door pockets: gloves, band aids, some tape, 2x2 and 4x4 pads, ibuprofen, a couple cravats, gauze. Easy to grab without digging through other stuff in the cargo area, and the stuff you would most commonly need.
The other is a larger version but still fairly basic, all the above stuff plus CPR mask, trauma and burn pads, more gauze rolls, more tape, emergency blanket, EMT shears, I think that's about it. I keep it in a velcro bottom pouch that was getting thrown away at the ambulance, after I cleaned up the velcro it stuck perfectly to the back of the back seats.
I don't keep it in my first aid gear, it's with my tools, but duct tape is a seriously great piece of kit. You can use it as a tourniquet, hold down dressings, wrap it around a stick to split an extremity, etc.
I used to carry a monster of a first aid kit with oral airways, a bag valve mask, IV supplies, the works. In almost 20 years I never used anything but the extreme basics - band aids, 4x4s, tape, and gauze. Maybe an ice pack. I decided between the lack of use and the potential for getting in trouble (starting IVs when not on duty, outside my area or even outside my state), it was just easier to carry the most simple stuff.
I suppose if I was taking an unsupported trip into no man's land I'd temporarily up my gear and include IV stuff, epi, pain meds if I could legally and easily source them, tourniquet, quick clot. That sort of thing. The IV and meds would only be for me or someone in my party.
Not really first aid but - don't forget a fire extinguisher.
