I've done this in 4.3, but it should be similar in older versions:
in include/search.php make the following changes:
1) Find these lines right near the top of the file:
"orderby" => func_get_langvar_by_name("lbl_default")
);
And right below that add:
# sort by manufacturer mod begins
if(!$manufacturerid)
$sort_fields["manufacturer"] = func_get_langvar_by_name("lbl_manufacturer"); #sort by manuf mod
# sort by manufacturer mod ends
2) At around line 630 there's a switch case. Right before these lines:
default:
$sort_string = "$sql_tbl[products].product";
Add:
# sort by manufacturer mod begins
case "manufacturer":
$left_joins['manufacturers'] = array(
"on" => "$sql_tbl[manufacturers].manufacturerid = $sql_tbl[products].manufacturerid"
);
$sort_string = "$sql_tbl[manufacturers].manufacturer $direction";
break;
# sort by manufacturer mod ends
3) Make sure to run
WWW.YOURSITE.com/cleanup.php
That's it!