N.M. Stat. § 4-41-7
In all cases of employment by county sheriffs of deputies, clerks and other personnel to positions covered by the merit system subsequent to the passage of an ordinance establishing a merit system, the contract of employment between the deputy or employee and the sheriff shall be considered to contain the provisions of the ordinance and all regulations issued pursuant thereto. The provisions of an ordinance and all regulations issued pursuant thereto shall become part of the contract of employment between the sheriff and all employees of the sheriff's office in positions covered by the merit system when the employment relationship exists at the time of the passage of the ordinance, unless the employee files with the county clerk, within ten days of the passage of the ordinance, a declaration stating that the employee does not desire to have the provisions of the ordinance, together with the regulations issued pursuant thereto, included as a part of his contract of employment.
NMS § 4-41-7