What should you do?

A. Build or leverage an OAuth-compatible access control system

B. Build SAML 2.0 SSO compatibility into your authentication system
C. Restrict data access based on the source IP address of the partner systems
D. Create secondary credentials for each dealer that can be given to the trusted third party

Explanation:
Delegate application authorization with OAuth2
Cloud Platform APIs support OAuth 2.0, and scopes provide granular authorization over the methods that are supported. Cloud Platform supports both service-account and user-account OAuth, also called three-legged OAuth.

References: https://cloud.google.com/docs/enterprise/best-practices-for-enterprise-organizations#delegate_application_authorization_with_oauth2
https://cloud.google.com/appengine/docs/flexible/go/authorizing-apps