I'd take a look at the rear heater.  If you're lucky, there is a leaky hose.
If not, you'll need a heater core.  The heater core can be bypassed by
plugging the two hoses going to it together using a 1/2 inch plastic plumbing
elbow until you can make permanent repairs.

Generally, you have to remove the carpet and sound insulation and give them a
good washing with the garden hose to get rid of the sticky antifreeze.

Tom
Illinois