The tuner module should have shipped with an extension cable just for this purpose. (See here for one possible problem with the extension cable if you decide to use it.)

If this still isn't long enough, extending the length of this cable is not recommended as it would tend to pick up noise if it were much longer.

Often, careful and creative placement of the tuner module, wires, and connectors will allow the items to fit behind most dashboards. Try to carefully route and place all of the wires rather than just cramming everything in.

If there is a problem getting the car's antenna cable to reach the tuner module, then it should be no problem to extend the antenna cable. It's the cable that goes between the tuner and the sled that shouldn't be lengthened.