div.innerHTML = "<textarea cols=50 rows=5 maxlength='200'
name='arr_product_parameter[" + obj_param_div + "]'
id='arr_product_parameter[" + obj_param_div + "]'
style='width:500px;border: 1px solid #a2e692; font: Arial, Verdana, Tahoma; font-size:18px; padding:0px 0px 0px 10px; outline: none;' value=\"\"></textarea>
<font id=\"counter_"+obj_param_div+"\" style='font-size:14px; font-family: Arial, Verdana, Tahoma; color: #081d05; text-decoration:none; font-weight:bold;'>Введено: </font>";
var obj = document.getElemenstByName('arr_product_parameter[" + obj_param_div + "]');
obj.onkeypress='ch_length_text_area(\""+obj_param_div+"\")';
obj.onchange='ch_length_text_area(\""+obj_param_div+"\")';
obj.onkeyup='ch_length_text_area(\""+obj_param_div+"\")';
obj.oncut='ch_length_text_area(\""+obj_param_div+"\")';
obj.onpaste='ch_length_text_area(\""+obj_param_div+"\")';
obj.oninput='ch_length_text_area(\""+obj_param_div+"\")';
onkeydown='ch_length_text_area(\""+obj_param_div+"\")';
ну примерно так, можно соотвественно сделать отбор элемента отобрать по 'name' а не по id, тогда использовать getElementsByTagName