The CustomerLists response group provides the WishListId for each customer listed in the response.
Sample Response (REST)
<?xml version="1.0" encoding="UTF-8"?>
<CustomerContentLookupResponse xmlns="http://webservices.amazon.com/AWSECommerceService/2004-08-01">
<OperationRequest>
<HTTPHeaders>
<Header Name="UserAgent" Value="Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1)"/>
</HTTPHeaders>
<RequestId>0JXXH0XCP50PWHJFXEQC</RequestId>
<Arguments>
<Argument Name="Service" Value="AWSECommerceService"/>
<Argument Name="AssociateTag" Value="[Your Associate ID Here]"/>
<Argument Name="CustomerId" Value="A2KEKKJ9CAC2KC"/>
<Argument Name="SubscriptionId" Value="[Your Subscription ID Here]"/>
<Argument Name="ResponseGroup" Value="CustomerLists"/>
<Argument Name="Operation" Value="CustomerContentLookup"/>
</Arguments>
</OperationRequest>
<Customers>
<Request>
<IsValid>True</IsValid>
</Request>
<Customer>
<WishListId>BUWBWH9K2H77</WishListId>
</Customer>
</Customers>
</CustomerContentLookupResponse>
The Request that Generated the Response (REST)
http://webservices.amazon.com/onca/xml?Service=AWSECommerceService
&SubscriptionId=[Your Subscription ID Here]
&AssociateTag=[Your Associate ID Here]
&Operation=CustomerContentLookup
&CustomerId=A2KEKKJ9CAC2KC
&ResponseGroup=CustomerLists