The Sub Battery (commonly called AUX Battery) is a 12AH AGM power sports (think motorcycle/ATV) battery.
If you have an Interstate Batteries distributor in your area, I'd remove the old battery and bring it to them. They'll buy your old battery for scrap and sell you a quality replacement for a reasonable price. I believe the Interstate MTX-AUX14 fits, and it's around $120.
If you want the genuine Mercedes replacement, (part # 0009829608), then FCP Euro sells it for $142.99, and they will ship it to you.
Alternatively, you should be able to find a suitable replacement at any auto parts store that carries motorcycle batteries. Even many Walmart locations carry them.
If you have an Interstate Batteries distributor in your area, I'd remove the old battery and bring it to them. They'll buy your old battery for scrap and sell you a quality replacement for a reasonable price. I believe the Interstate MTX-AUX14 fits, and it's around $120.
If you want the genuine Mercedes replacement, (part # 0009829608), then FCP Euro sells it for $142.99, and they will ship it to you.
Alternatively, you should be able to find a suitable replacement at any auto parts store that carries motorcycle batteries. Even many Walmart locations carry them.