Have you looked at what gets sent over the wire? I find the HTTP Live Headers plugin for FireFox quite useful but Wireshark is also good for monitoring what actually gets sent.

It seems to me that xMakeID gets set to "32" and MakeID to "32" as well, so all should be good, but in the end what actually gets sent is what matters :) Maybe there are some additional parameters that get set by JavaScript that you haven't found out about yet.