The cheap, lazy, and might work good enough method is to ground it to bare metal. (If you have enough of a system that you need a second battery, you're probably past this).
The best way is to do it to the frame. You could drill through metal in a good spot, and use something like this:
Amazon product ASIN B0741H211X
I would bet that under the carpet there are drain holes. That are plugged with a removable plug if you don't want to drill through metal, and want to be able to go back to factory if you ever sell it. (Pop underneath and you can probably see em before pulling anything up, and should get a good idea if it's worth taking out the interior to get to em).
I used:
Amazon product ASIN B07QKPHMH1&
Amazon product ASIN B09B957KXX
Needed to take up a little slack and cut the head off some zip ties, and fed it between the two.