admiral,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<script>
document.addEventListener( 'DOMContentLoaded' , function() {
const on = (parent, event, selector, fn) => parent.addEventListener(event, ({target}) => {
if(target = target.closest(selector)) fn(target)
});
const makeSelector = '[name="stm_f_s[make]"]';
const make = document.querySelector(makeSelector);
const serie = document.querySelector('[name="stm_f_s[serie]"]');
const changeEvent = _ => make.selectedIndex ? serie.removeAttribute('disabled') : (serie.setAttribute('disabled', 'disabled'), serie.selectedIndex = 0);
changeEvent();
on(document, 'change', makeSelector, changeEvent);
});
</script>
</head>
<body>
<select class="add_a_car-select select2-hidden-accessible" data-class="stm_select_overflowed" data-selected="" name="stm_f_s[make]" tabindex="-1" aria-hidden="true">
<option value="" selected="selected">Select Make</option>
<option value="">1</option>
<option value="">2</option>
<option value="">3</option>
<option value="">4</option>
<option value="">5</option>
</select>
<select class="add_a_car-select select2-hidden-accessible" data-class="stm_select_overflowed" data-selected="" name="stm_f_s[serie]" tabindex="-1" aria-hidden="true">
<option value="" selected="selected" >Select Model</option>
<option value="">1</option>
<option value="">2</option>
<option value="">3</option>
<option value="">4</option>
<option value="">5</option>
</select>
</body>
</html>