The code added to Weapon.ini should only provide the weapons, but it will not bind the weapon to the unit being replaced, unless all you did was replace
parts of the weapons code
I assume you're replacing the model for the Chinese Inferno Cannon in which case, the following should be what you need:
1. Extract ChinaVehicle.ini and place it into Data\INI\Object
2. Open it up and look for the entry on the Inferno Cannon
3. For all assignments of "Model" in the entry for the Inferno Cannon, replace the model name with the model you downloaded (without the .w3d)
4. Be aware that there are multiple "ConditionState"s, each representing each state the unit is in. Make sure the correct model is allocated to each state.
5. If you added a weapon into Weapon.ini, find the "Weapon" assignments and replace them with the correct weapon names that you added into Weapon.ini
This
should work if I remember correctly and didn't forget anything. I haven't modded for a while so I'm working mostly from memory of what I did back then right now.
Notes: When you extract a file from any of the .big files, if you edit them, do not put them back into the .big files. As long as the individual files are in the right place, the game engine will recognize these instead of the original ones in the .big file.