Here is the code for creating the link to Mapquest. The field name it uses for the post code is "Pcode", ewither rename the zip code field or create a new one, either way, ensure it is in this format or it will not work. Of course, you could alway amend the code below to reflect you field name.
I cannot take credit for the whole site, it is based upon a template found here and ALL of the features/mods have also come from this forum. I have done is used the code freely abvailable and put it together.
<a href="http://www.mapquest.co.uk/cgi-bin/ia_find?link=btwn%2Ftwn-map_results&random=1167&event=find_search&uid=u8na _7e2slm3sapa%3A25lrt29y80&country=Great+Britain&SN VData=&address=&city=&Zip=<?php renderSingleListingItemNoCaption($listingID, "Pcode") ?>&Find+Map.x=35&Find+Map.y=6" target="_blank"><strong>Look at a map of the area</strong></a>