How to create a custom device tracker (from GPS coordinates)?

Configuration
9

I tried the solution but it doesn’t seem to work. No tracker created.
Also, when trying to use the service manually I encounter a host of issues, basically the format for gps doesn’t seem to be valid for device_tracker.see call.
Any input?
This is what I am trying to do:
Sensors for coordinates extracted from MQTT by my Leaf OVMS:

   - platform: mqtt
     name: leaf position lat
     state_topic: "ovms/nissanleaf/xxx/metric/v/p/latitude"
     qos: 1
   - platform: mqtt
     name: leaf position lon
     state_topic: "ovms/nissanleaf/xxx/metric/v/p/longitude"
     qos: 1

Automation to update the position/create the tracker:

- id: '1603005245535'
  alias: 'Update Leaf position'
  description: ''
  trigger:
  - platform: state
    entity_id: sensor.leaf_position_lat
  - platform: state
    entity_id: sensor.leaf_position_lon
  condition: []
  action:
  - service: device_tracker.see
    data:
      data_template:
        dev_id: Leaf
        gps:
         - "{{ states('sensor.leaf_position_lat') }}"
         - "{{ states('sensor.leaf_position_lon') }}"
  mode: single

This is actually triggered (I’ve checked) but no tracker is ever created.
I tried triggering the method manually using the developer tools but it complains about the data_template not being a valid keyword or that I am missing dev_id (depending on how I build up the yaml).