When looking in my engine department on my '99 4Runner, I noticed that on the passenger side (right side - RH) that there is a very similar configuration that already exists for battery on the stock/orig driver's side (left side - LH).

After doing some research, I found that many foreign countries that offerred a diesel engine also offerred a dual battery set-up as an option. Did some further research and found-out that all the parts are available.... but the bracket itself (also known as "Carrier Sub Assy") is ONLY available from Japan.
Not cheap (approx $60 shipped to your door) but wanted factory Toyota part for the RH side.
Arrived in just 10 days! (Japan to Los Angeles) http://japanparts.com/

Here are the Toyota Part numbers:
Batt Hold-Down Bracket (RH) - 74403-35010 only avail in Japan
Batt Hold-Down Bracket (LH) - 74404-35140 stock bracket - avail in US
Long Hold-Down Bolt (same LH & RH) - 74451-35060 avail in US
Nut for long bolt (same LH & RH) - 90179-08220 avail in US
Small bolt w washer (same LH & RH) - 90119-08974 avail in US

This route to add an extra battery is super easy for my diesel 4Runner... everything just bolts right in! BUT, after looking at my sister's '99 in Calif, it looks like for 4cyl USA versions the airbox would need to be moved/modified or discarded to be able to mount a battery in this second factory location. (No idea about 6cyl gas 4Runners)

Hope this saves someone some time.... took me awhile.

 

Nice find; you should post up pics of your finished install. The loss of the airbox would be problematic for us US folks. Our best bolt-on solution would probably be from Columbia Overland ($150) which mounts two batteries as well as relocates the overflow tank.

 

Yep.... I've seen that option. Looks very clean and functional.
I really wanted to go the factory route for two key reasons:

1) Weight Distribution - with driver and battery already on left side, I wanted to put the weight of an extra battery (about 40lbs) on the passenger side instead.
2) Complete bolt-on using factory parts & mounting locations. (But as mentioned prior, I don't need to worry about relocating anything vs. USA 4Runners)

This is a work in-progress... not a super high-priority until I get a winch or a new subwoofer/amp which will draw more from batt when I'm parked. So no finished pics for awhile. But to tide people over, I'll work on posting a pic of the nice new RH bracket (almost a mirror image of the LH factory bracket).

 

Could you take a pic of how your engine bay is set up differently than our?

 

I'm still in Calif until another week..... but will be returning to Nicaragua (where my 4Runner is) and will take a pic then. My impression is that ALL '99 4Runners have the exact same sheet metal... only thing that differs is how Toyota locates some things (chiefly the air filter housing). Globally at least three engines exist that year: 3.0L Diesel, 4Cyl gas, 6Cyl gas.

Will take a pic soon to show-off my roomy engine compartment!

 

Here are two pics of the factory Toyota bracket ordered from Japan:





This fits the same size battery as what comes standard in the 4Runner.
(Same exact "clamping" size as the original hold-down bracket on the LH or US Driver's side)

 

My 99 Surf (4 Runner Diesel) has a Dual Battery positioned on opposite sides of the car. Would you like photos ?

 

Yes, please!

 

Nicely designed, but way too much weight on such a flimsy area. Sheet metal supporting 100+lbs? Recipe for a ruined inner fender.

 

As requested, here are two photos of my 3.0 L Turbo Diesel engine
compartment. This is completely stock/original... nothing has been moved.

You can see my battery bolted-in on the driver's side. Also note the big open
space on the passenger side (RH) between the large round airfilter housing and the radiator/grille. That's virtually the same "base" design as what the driver's side battery sits on.

Shame that the US trucks have their airfilters mounted right over this spot. My bet is that any Stateside truck could be fitted with this design (assuming you can source the foreign filter housing). I know for sure that the filters themselves can be bought in the States (cheaper than Nicaragua dealers too!).