There are sometimes 1 or 2 buses not in use in the fuse box, at least on the W123s and I would guess the 126s as well. If the non-use bus is dead, you could run a jumper of 16 or 14 guage from the aux-fan bus (hot side) to the dead bus and in turn, a lighter wire to the cabin. Directly from the battery would be a bit messy.