You need to hire a family law attorney who is familiar with guardianships. The guardianship application process is not easy, and would be extremely difficult for a non-attorney to navigate.
Since the grandparents do not have custody of the child in question, they do not have the right to agree or not to agree to the guardianship. Although the mother apparently does not want to either care for the child or to grant guardianship to you, you may be able to obtain guardianship over the minor child. Also, as the child is 16, her wishes are taken into account by the court. Guardianships cases begin with the filing of a Petition for Appointment of Guardianship of Minor in the Probate Court. I urge you to contact an attorney in your county with experience in Probate and Guardianship cases. I have included a link to San Joaquin County Bar Association's Referral Service. If you require low cost legal assistance, they can probably refer you to someone.