Performing a Lookup from a Zip Code

When a new Customer record is saved in the system, we'd like to assign it to a Media Market based on the Customer's Zip Code.  For example, Zip Code 10017 = Media Market New York.

We've adding this function to the Save Record function on the Customer form.

We created a Custom Record Type for the list of 3-digit Zip Codes for each Media Market.

We're having trouble referencing the Zip Code on the Customer record and having never done a lookup before.  Here is our code.  What are we doing wrong?


function getMetroMarket()
{
var zipcode;
    var ziplookup;

    /* get reference to Customer's Zip Code */