I've changed the template into:
http://www.imcdb.org/vehicles_make-{{{make}}}{{#if:{{{model|}}}|_model-{{{model}}} }}.html {{{1|{{PAGENAME}}}}}

If someone is interested, here is a list of pages where the two templates might be usefull:

http://en.wikipedia.org/w/index.php?title=Special:Linksearch&target=www.imcdb.or- g&namespace=&limit=500&offset=0