The chassis you use probably has a script that either sets the position directly or un-anchors the base when the game runs. Anchored parts are not affected by the physics system whatsoever, and anchoring the base won’t solve the issue of the hinges not connecting.
I recently tried to make a vehicle using the old method (like @oryRBX was doing) and it wasn’t working for me either. So I just followed the new tutorials on the dev hub and it worked.
Make sure everything is unanchored! Also, try welding it. I have seen many videos on YouTube teaching you how to do this, plugins like F3X have weld features, videos like this are helpful;
I have the same issue with trains, I have to write a function to MakeJoints() now. I used to be able just to insert it but now the wheels just fall off.
This works for me. Remove the script from the model and delete the brick. Place the script into the car model and it will make joints to keep the car from falling apart.
As you know, Roblox does not do automatic welding of vehicles anymore. So I recommend you use this script while having no models inside of the model you are working with:
local Car = script.Parent
Car:MakeJoints()
script:Destroy()
Sorry if I am two years late to the party. If anyone stumbles upon this, place this script into your car.