for one you need to change $2- into $1- also no need to set a var

i would do this instead

alias google { if ($1) { url $+(http://www.google.com/custom?q=,$1-) } }
alias youtube { if ($1) { url $+(http://uk.youtube.com/results?search_type=&search_query=,$1-) } }
alias wiki { if ($1) { url $+(http://en.wikipedia.org/wiki/special:search?search=,$1-) } }