custom taxonomy and wp_set_post_terms

please help!!!!!!!

I am trying to set the post custom taxonomy via two variables ( parent Id and child Id) that I’ve gotten from another function in my plugin file. var’s $a and $b

when i manually enter the term ID – everything works fine
but when I try to use a variable – to set the term id – it dosen’t work

function for getting the id’s in the first code snippet, and trying to save the terms in the second snippet.

//***IF YOU HAVE MODEL GET THE ID

if(!empty($auto_model)){

$auto_model_id = get_term_by( 'slug', $auto_model, $this->tax );
$auto_model_id = $auto_model_id->term_id;

} // end get model id

if ($auto_make_id !== null && $auto_model_id !== null) { // +++++ IF YOU HAVE BOTH 

$this->p_id = $auto_make_id; // define variable
$this->c_id = $auto_model_id; // define variable  

// set post with both these terms - jump to set post and pass ids to set post 

//return $p_id;
return $c_id . $p_id;

}

function cicb_do_things($post_id) { // use this as the final build up before sending to construct
global $auto_make,$auto_model; 

$this->makemodel();

    $a = $this->p_id;

    $b = $this->c_id; 

$tag = array( 45 ); // this is how wp wants me to do it; and when I do the wp_set object works great

var_dump ($tag);

echo '<br>';

$tag = array( $a ); // when I put in my variable which is the also 45 - the term does not work 

wp_set_object_terms( $post_id, $tag, $this->tax );
}

what am I doing wrong? pls. help. been at this for more than a week now.