I believe you have encountered the classic "many to many" relationship challenge. Any different customer can have multiple service locations and any given service location can be linked to multiple customers.
If this is correct, you need a third "join" table so that you can link to many records when starting from either CustomerLocations or ServiceLocations.
CustomerLocations::CustID = Customer_ServiceLoc::CustID
ServiceLocations::SvcLocID = Customer_ServiceLoc::SvcLocID
You can place a portal to Customer_ServiceLoc on a customerLocations layout and include fields from ServiceLocations inside the portal row to list service locations for that customer. You can place a portal to Customer_ServiceLoc on a serviceLocations layout, include fields from CustomerLocations and use it to list customers for that service location.
Here's a demo file you may want to down load and examine. It matches Companies to Contracts, but if you renamed the tables to match what I posted up above, it would work for your as a starting point for this concept.
Thank you Phil. After a lot of scratching my had I got it to work in a Customer -----<Customer Service Locations ----------- Service Locations type of relationship.
But I think that many to many relationship is a better way to go (sometimes one customer pays for one service and another for other and it is the same location)
Thanks for the link. I will have a look.