ACF + Rest API = Powerful!

step3A-functions.php

<?php
//REST API - Enable more fields to be searched via Rest API
add_filter( 'rest_query_vars', function ( $valid_vars ) {
    $update_vars = $valid_vars;

    //Vehicle Meta Data
    $update_vars = array_merge( $update_vars, array( 'make', 'meta_query' ) );
    $update_vars = array_merge( $update_vars, array( 'model', 'meta_query' ) );

    return $update_vars;
} );

step3B-functions.php

<?php //REST API - Vehicle - Add make and model as a filterable parameters
add_filter( 'rest_vehicle_query', function( $args, $request ) {
    $make   = $request->get_param( 'make' );
    if ( ! empty( $make ) ) {
        $args['meta_query'][] = array(
                'key'     => 'vehicle_make',
                'value'   => $make,
                'compare' => '=',
        );
    }

    $model   = $request->get_param( 'model' );
    if ( ! empty( $model ) ) {
        $args['meta_query'][] = array(
                'key'     => 'vehicle_model',
                'value'   => $model,
                'compare' => '=',
        );
    }

    return $args;
}, 10, 2 );

step4-functions.php

<?php 
//Both call admin and front-end call the same JS file with same nonce

//ACF - Vehicle - Enqueue scripts for REST dynamic management of fields on front-end
add_action( "wp_enqueue_scripts", "enqueue_vehicle_rest" );
function enqueue_vehicle_rest() {

    if( is_singular( 'vehicle' )):
        wp_enqueue_script( 'vehicle-rest-edit', js/vehicle_rest_edit.js', array( 'jquery', 'underscore', 'acf-input') );
        wp_localize_script( 'vehicle-rest-edit', 'rb_vars', array(
            'rb_nonce' => wp_create_nonce( 'wp_rest' ),
        ));
    endif;

}

//ACF - Vehicle - Enqueue scripts for REST dynamic management of fields on admin interface
add_action( 'admin_enqueue_scripts', 'enqueue_admin_vehicle_rest' );
function enqueue_admin_vehicle_rest( $hook ) {

    $type = get_post_type(); // Check current post type
    $types = array( 'vehicle' ); // Allowed post types

    if( !in_array( $type, $types ) )
        return;

    wp_enqueue_script( 'vehicle-rest-edit', js/vehicle_rest_edit.js', array( 'jquery', 'underscore', 'acf-input') );
    wp_localize_script( 'vehicle-rest-edit', 'rb_vars', array(
            'rb_nonce' => wp_create_nonce( 'wp_rest' ),
        )
    );

}

vehicle_rest_edit.js

(function($) {
    //Customize the filter process for select fields where dynamic REST calls are required from one or more fields
    acf.add_filter('select2_args', function( args, $select, settings ){

        //Vehicle select
        if( settings['key'] == "field_577bee5ca4141") {

            if (args['ajax']) {

                args['ajax'] = {
                    url: "https://autosearch.dev/wp-json/wp/v2/server",
                    dataType: 'json',
                    type: 'get',
                    cache: false,
                    beforeSend: function (xhr) {
                        xhr.setRequestHeader('X-WP-NONCE', rb_vars.rb_nonce);
                    },

                    data: function () {
                        //List is based on the vehicles selected make and model
                        var data = acf.prepare_for_ajax({
                            make: $("#vehicle_make input").val(),
                            model: $("#vehicle_model input").val()
                        });

                        return data;

                    },

                    //Parse WP REST API data out into post ID and slug for display in select
                    results: function (data) {
                        if (!data) return [];

                        var output = [];
                        $.each(data, function (i) {
                            output[i] = {
                                id: data[i].id,
                                text: data[i].slug
                            };
                        });

                        return {
                            results: output
                        };
                    }
                };
            }
        }

})(jQuery);