Heater rebuild? Never heard of that. Here's what I'd check:

Heater core:
* Leaks (before removing, check for leaks in passenger area, under carpet, on firewall cardboard, etc)
* Flow (ensure it is not plugged). You can test similar to radiator blocked test
* Clean out debris in exterior of core

Hoses: Check for cracks, overly soft hoses, tight clamps

Flow valve: ensure it opens & closes easily through entire range. Also check linkage for easy and complete movement (before taking apart)

Air ducts: Ensure they are clean and connected properly

Electrical:
1. Ensure fan spins at both High & Low speed