Q: On a 2007 VW GTI, if the starter solenoid connection wiring plastic boot clip is broken what is needed to repair it?

If the plastic you are referring to is simply a covering over a metal terminal, then a field fabricated repair can be effected and that would be something such as heat activated shrink wrap or weather resistant electrical tape. In the foregoing application, the plastic just represents insulation. You used the term "switch" though. If a plastic switch or plastic solenoid housing is cracked or missing, the part has to be replaced. Plastic in that application has a mechanical or structural function. In all events, what you are describing is repairable. If the issue is you need a new starter motor, you can request starter replacement. If the vehicle is not starting, but you first want a complete and conclusive diagnostic, please instead request a no start diagnostic. In either case, the responding certified mechanic will make the necessary repairs and get the car running for you. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.