You'd treat the repair centers like suppliers (of a "repair" service).
As such, the requirements are for you to define, presumably based on some sort of risk-assessment and any applicable regulatory requirements (e.g.
FDA 21 CFR 820.200)
In an agreement you can dictate such things as:
- The scope of repairs they can perform
- The established procedures that must be followed when conducting repairs
- What equipment & parts they can use, and where they get them from
- Required training/qualification of repair technicians
- Required post-repair testing
- Records they are required to maintain (you may stipulate that these are sent to you, or how they are maintained/reviewed)