Hi,
To become a Lead Auditor for TS 16949, you need to be an employee of a certification body like BVQI, UL, TUV, DNV, etc. They will nominate you on behalf of them, then only you are eligible for the qualification. The qualification, if done by IATF, is mainly SMMT or AIAG. Visit the website
www.iaob.org.
To become a lead auditor for other products, you can apply to any certification bodies that are conducting lead auditor courses. I hope this may help you.