According to the IATF 16949 Clause 7.2.3 (Internal auditor competency), there are not any requirements to have internal auditors such as a lead auditor in automotive industries.
In my experience, External auditors are not logical to make the internal process and product audits. Sometimes the organization can want to check itself from external auditors to take objective thoughts. It can help because sometimes internal auditors can be blind or can't see to catch the unconformities.
Also, the organization can receive training from the external auditors. To give motivation and information for the employees, taking training from outside can be effective to increase auditor qualifications.

However, the organization should define an internal audit procedure, checklists, and internal auditors. Every time the organization cannot take supports from the external auditors so every department owner should be able to check themselves according to the IATF requirements.
The QMS departments must already know all requirements. When the external auditors visit to assess the product or process, who will check their efficiency? Of course, QMS departments. The organization should educate qualified auditors internally. In the organization, employees can only know well all steps about the process and product. Manufacturers who are in constant contact with the customer are the ones who know the produced part, process, production line, system, process, deficiencies, pros, employees, and of course customer expectations. If the organization has a lead auditor, they can share their experiences with employees such as technical training, product training, core tools, applied audit training, customer expectations, and standard training.
According to the IATF 16949 Clause 7.2.3 (Internal auditor competency):
That's why Quality management system auditors, manufacturing process auditors, and product auditors shall all be able to demonstrate the following minimum competencies:
1. To understand the automotive process approach for auditing, including risk-based thinking;
2. To understand customer-specific requirements, ISO 9001 and IATF 16949 requirements related to the scope of the audit;
3. To understand core tool requirements related to the scope of the audit;
4. To understand how to plan, conduct, report, and closeout audit findings.

Especially, the manufacturing process auditors shall demonstrate a technical understanding of the relevant manufacturing process(es) to be audited, including process risk analysis (such as PFMEA) and control plan. Also, Product auditors shall demonstrate competence in understanding product requirements and the use of relevant measuring and test equipment to verify product conformity. Where training is provided to achieve competency, documented information should record the trainer’s competency with the above requirements by the organization.

To maintain and improve internal auditor competences, the organization should execute a minimum number of audits per year, should define and maintain knowledge of relevant requirements based on internal changes, and external changes.

JOURNAL OF AN AUDITOR