Open OpenIV, turn on Edit Mode
Go to: mods/update/update.rpf/common/data

In case you've already installed handling mods before which replaced the handling.meta, go to the same location, and follow the Manual method.

Automatic Method:
Drag and drop my "handling.meta" into OpenIV

Manual Method:
Mouse right click on game's "handling.meta", and click "Edit"
Click on the search/On keyboard, press Ctrl+F, search for "Cavcade", and then open my "Cavcade.txt", copy paste the whole text into the game's handling meta Cavcade section, starting from "<Item type="CHandlingData">" till "</Item>"
Repeat the same for the "Mesa"

Now you have an option to install my mod on either of the vehicles using the manual method. 