Lets see you dual battery setups!
 

I ended up buying a new battery, thought the old one was toast.

So figure why not add the old battery!

Both stock size batteries.

Thinking adding the second battery to cargo area (rear mount spare).

Thoughts on placement?

Share your setup please!

How would I wire it up?

Thinking just add ground to any good ground point in cargo area and run a positive cable to engine bay.
Connect directly to front battery pos+ terminal?
4 gauge? thinking similar to adding an amp?
Fuse connection?

Alternatively considered tossing stock intake and making custom intake and add pass side battery tray?
Seems like a lot of work just to mount an extra battery, lol

Or any ideas what to do with a massive 12 batt? lol
unrelated projects?

put the battery on a hand truck add some booster cables and use it as a mobile booster pack