Albert:
All four exhaust ports ( 2 front, 2 rear) must connect to the muffler. If you still have your rear sections, consider having the exhaust pipes repaired and find better ones with the heater boxes in the future. You don't want to cap-off any of the exhaust ports. If I understand you correctly, you need either new heater box assemblies or at least a pair of replacement J-tubes. The link below is for photos - I don't think our host sells these.
Porsche 912 / 912-e Exhaust – TT Exhausts
Bill