The metal lines for my rear heat have rusted thru. I want to run rubber hose as a temporary fix.

What size hose fits the barb under the front passenger seat?

if i recall it is 3/8", but you could just check in the engine bay as it would be the same barbed nipple and easy to access, I hear that the 2000+ got SS rear heater lines that should still be good if you find a donor at the wreckers, another option i've seen people use is copper 3/8" propane/NAT gas lines, flexible and should also last the life of the vehicle.