Good radiator shops can fix the rear heater cores, if they are fixable at all. I had 3 cores done at one shop for $100 or so. Some shops won't agree to work on these, but some will - you need to carry in the heater core (if it's remotely fixable) and talk them into it.
Steve H. - KE0KNH
Colo Springs, CO
'78 FJ40, '89 FJ62, '98 4Runner